Thickening and strengthening epoxy

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T34zac

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Right now I'm building a Madcow 4" Frenzy XL. Unfortunately I am almost out of my trusty proline 4500 epoxy. I do however have a lot of west systems 105/205.

My question is what techniques does everyone use to thicken and strengthen this epoxy for use as a structural rather than layup epoxy?

(Yes I did use the search bar, it doesn't work)
 
Depends what you are going after in your fillets. If you are really going to push the limits of the kit, use a filler that gives max strength. If you want weaker (not that weak) but sandable fillets, use micro balloons. The big epoxy vendors have selection guides https://www.westsystem.com/ss/filler-selection-guide/

Pushing the kit is not something that I would do for a while. I really want something to make it strong enough to be comparable or better than the Proline 4500.
 
I just use proline 4500 or rocket poxy for external fillets. For internal I use west system or US composites with chopped carbon fiber.
 
Follow the link that Dick gave to West Systems.

I use 406 for most thickening where I want strength. 410 for light weight and easy sanding for more decoration than strength.
 
The problem is that if I'm ordering something, it's going to be more proline 4500. I just wanted to know if there was anything that I could find kicking around my workshop that would work to strengthen the west systems so I could get started on my fin can.
 
I wouldn't count on anything in your shop being as good as bought fillers. I have heard of people making their own from cut up fiberglass cloth but I doubt you can make it as uniform as what you buy. Does Proline offer their own fillers?
 
406 colloidal silica. [cabosil] You can usually find this at West Marine stores in every major city/mall or hobby shop under the "house" brand of filler. It's just fumed silica, light as feathers, but adds bonding strength to fillets. I know you what you stated about ordering , but maybe you do have it for other hobbies or drive right by a West or fiberglass supply type store [boats or car guys use it.]

I use it all the time, in various epoxies. Really cheap [20 bucks for big container] and handy to have around for all it's properties.
 
+1 for marine stores. Before I knew there were super duper rocket friendly epoxies I got West epoxy and fillers from a local marine store. I didn't even know they were around until then. Being a non-radical rocketeer, I prefer Superfil for my fillets. Basically epoxy premixed with a light filler to make it sandable and light.
 
I am a fan of using a proper adhesive epoxy rather than thickening a laminating epoxy, particularly West Systems which I never even liked all that much for boats. You will never get as consistent a mix with your fillers as the manufacturer will, and you end up getting clumps, air bubbles, etc.

Proline 4500 makes beautiful fillets but that beauty comes at the price of being somewhat brittle (seriously - cure a dollop of the stuff and then hit it with a hammer, but wear safety googles). RocketPoxy and AeroPoxy ES6209 are both very good adhesive epoxies, and RocketPoxy is more convenient for fillets. I find myself using RocketPoxy more and more.

The only downside to RocketPoxy is that, while it has a generally inoffensive smell by itself, when combined with the denatured alcohol used for cleanup it produces an odor most foul. But it's still the epoxy of choice for most things in my shop.
 
I am a fan of using a proper adhesive epoxy rather than thickening a laminating epoxy, particularly West Systems which I never even liked all that much for boats. You will never get as consistent a mix with your fillers as the manufacturer will, and you end up getting clumps, air bubbles, etc.

Proline 4500 makes beautiful fillets but that beauty comes at the price of being somewhat brittle (seriously - cure a dollop of the stuff and then hit it with a hammer, but wear safety googles). RocketPoxy and AeroPoxy ES6209 are both very good adhesive epoxies, and RocketPoxy is more convenient for fillets. I find myself using RocketPoxy more and more.

The only downside to RocketPoxy is that, while it has a generally inoffensive smell by itself, when combined with the denatured alcohol used for cleanup it produces an odor most foul. But it's still the epoxy of choice for most things in my shop.

I have some rocketpoxy left over from my last 4" rocket. I can bear the smell, but I hate how difficult it is to work with. While it may produce some good fillets, it is just hard to mix and apply, maybe I got a bad batch?
 
I have some rocketpoxy left over from my last 4" rocket. I can bear the smell, but I hate how difficult it is to work with. While it may produce some good fillets, it is just hard to mix and apply, maybe I got a bad batch?

It is quite thick. I put the part B stuff (brown) in a use ketchup squeeze bottle and it helps a lot.

Once mixed, I don't think it is excessively thick. The part A stuff alone is hard to deal with, though. And I think it makes up for those difficulties with its easy 1:1 mix ratio and tolerance of the old equal-glob measuring method.

When I first got it I didn't use it much for the same reasons, but I have found that the more familiar I become with it the more I like it.
 
It is quite thick. I put the part B stuff (brown) in a use ketchup squeeze bottle and it helps a lot.

Once mixed, I don't think it is excessively thick. The part A stuff alone is hard to deal with, though. And I think it makes up for those difficulties with its easy 1:1 mix ratio and tolerance of the old equal-glob measuring method.

When I first got it I didn't use it much for the same reasons, but I have found that the more familiar I become with it the more I like it.

Even with measuring the proline I find it much easier to use. But I may get the rocketpoxy going again for the centering rings.

I don't think I'll every use a different epoxy for fillets though. Proline produces such a nice fillet, I just can't use anything else.
 
Leave the West System to laminating; RocketPoxy and Proline to fillets... In my book the best structural epoxy hands down is System Three T-88: https://www.systemthree.com/store/pc/T-88-c27.htm.

T-88 is the best- expensive but really good. I have repaired a lot of canoe paddles and I would not use anything else. However, none of the rockets I build require that level of strength. For my fillets I use 5 min epoxy mixed with a thickener. I also mask for the fillet.
 
Even with measuring the proline I find it much easier to use. But I may get the rocketpoxy going again for the centering rings.

I don't think I'll every use a different epoxy for fillets though. Proline produces such a nice fillet, I just can't use anything else.

I know, Proline make 'em pretty, but it's just so brittle when it cures. I am of the school of thought that says that everyone since Stine has misread Hoerner's book on drag when it comes to fillets, so I try to keep the external ones as minimal as possible (where Proline's tendency to crack doesn't matter) and use the RocketPoxy internally for strength.
 
I know, Proline make 'em pretty, but it's just so brittle when it cures. I am of the school of thought that says that everyone since Stine has misread Hoerner's book on drag when it comes to fillets, so I try to keep the external ones as minimal as possible (where Proline's tendency to crack doesn't matter) and use the RocketPoxy internally for strength.

Not quite sure how I'm going to get rocketpoxy inside. It's way too thick to inject and it just seems that 'buttering it on' would end in too weak if a bond
 
I use talc baby powder on occasion thicken epoxy. Surely you got some of that laying around. Im not sure how strong it is. I have yet to hammer a rocket with large motors. I have never used the epoxies you mentioned either so don't know how it stacks up against those. Wouldn't hurt to try a small bit and test it. The Talc is inert and doesn't react in any way with the epoxy. It is being considered as a substitute for Portland cement if I remember right.

Mike
 
Kevlar pulp is you friend. You do not sand the fillets directly though. Once in place use an epoxy based sand able filler and sand that.
 
Some quick facts: West system epoxy is basically an unfilled epoxy made as a laminating epoxy I am not knocking it but just want you and others that may read this to know some of the differences of the epoxies being talked about on this thread. West system would be good for laminating fiberglass cloth to a plywood fin or fiberglass cloth to a cardboard tube or something similar. That is why West system epoxies have a thin viscosity of less than 1,000 centipoise, so the epoxy can flow around and into the fiberglass cloth, that is why they are called laminating epoxies. If you are purchasing a kit that has a filament would tube with G10 fins this would not be near as good a choice as using a filled high strength epoxy because you are not laminating the fiberglass cloth or fiberglass thread into a tube, the filament wound tube and G10 sheet manufacturer has already done this for you, you are at this point needing a high strength structural filled epoxy to assembly the tube and G10 fin and components for the best results. To use an unfilled epoxy similar to West System you would first need to add lots of fillers and thickening agents to make it anything even close for bonding strength to a filled high strength structural epoxy such as Rocketpoxy G5000. Rocketpoxy G5000 is in an epoxy classification called filled high strength structural epoxies. That is why Rocketpoxy has a viscosity of 350,000+ centipoise (basically a paste). If you love to formulate epoxies and have the time and knowledge to purchase, add, and mix the correct amounts and types of fillers needed to make a superior high strength filled epoxy then the west system unfilled epoxy is for you, if you want a high strength structural filled epoxy to use right out of the jar where an expert chemist with lots of test data has already done all the hard work for you by adding and mixing in the exact formulation of fillers then use something like the Rocketpoxy G5000. That is why filled epoxies do have better physical properties than unfilled epoxies for structural assemblies such as filament and carbon fiber wound tubes, G10 and carbon fiber sheet products, metals, plastics and generally any composites. Most serious composite assemblers that wish to use an epoxy ready to build with will prefer them, also that is why filled epoxies may cost a little more than an unfilled epoxy.

Also when comparing Rocketpoxy to an epoxy such as the Bob Smiths 20 or 30 minute epoxy or Proline 4500 the Bob Smith and Proline would be more brittle and have less bonding strength with some composite materials especially plastics and aluminum. Yes one of the major tradeoffs of a higher temp and faster setting epoxy is that they generally become more brittle. We feel that Rocketpoxy G5000 is the perfect tradeoffs of high strength, not brittle, curing time, and a decent temperature rating that is good for about 98% of all high powered flights. You may have a little more of a learning curve with the Rocketpoxy and may have to be a little patient about the curing times but it will give you a great finished product where lots of fin flexing and even some hard landings will not crack the epoxy. I’ll give you a quick test you can do put about ¼ - 3/8 inch layer of your West system, Proline , and Rocketpoxy G5000 epoxy into a plastic cup and let it harden, then whack all your samples with a hammer, you will notice quickly which epoxy did not shatter like brittle glass ( hint the RP G5000 will not easily shatter) , you will also notice which epoxy bonding strength stuck to the plastic cup and which epoxies ere easily popped off (you will not be able to separate the Rocketpoxy G5000 from the cup). You can have a higher temp and faster setting epoxy but if it is very brittle and less bonding strength this may cause failure well before any temperature comes into play. I’m not trying to get all the West systems, Bob smith and Proline users panty’s in a bunch, all these epoxies have their place and I sure as well know how this forum can get at times and not looking into getting into a big back and forth with this. If it works great for you more power to you, I’m just trying to answer a few questions I have been asked about this. Since my company is the Manufacturer of Rocketpoxy I do feel it is important to get the facts straight from the manufacturer who has actually done a lot of lab work and test data on this.
 
Quick comment I have on questions I have been asked on injection internal fillets, I personally don't like drilling injection holes in my FW tubes (making stress concentration holes right next to the slots doesn't do it for me) plus it wastes a lot of time to drill these, also trying to make sure everything inside is "leak proof" you know where every part has to be butted up together so no epoxy leaking though the motor centering rings can occur is difficult, tedious and time consuming and do not find any advantages to using the “injection" method, also I like to be able to see the internal fillets when finished, with injection method using black or colored FW tube this is not possible. I think this was originally developed because it was basically the only way you could do it when using the thin runny epoxies, I find it much easier to simply use a 1/8” to ¼” diameter wooden dowel to build my internal fin fillets laying the rocket horizontal and going in through the bottom and attaching the motor mount rear ring on afterwards, really this is pretty fast once you get the hang of it, you simply just dip the end of the dowel into the mixed Rocketpoxy twirl it a little to build up on the dowel, then go in the aft side of the rocket and twirl the Rocketpoxy into the fillet locations, you can see the internal fillets when finished and make them exactly how thick you want them, no guessing. I have even done minimum diameter rockets this way as well. I do four fillets per 120 degree rotation (for a 3 fin rocket), the two internal fillets on the bottom single fin, and the top two internal fillets on the two top fins per rotation, (or if you really want to speed things up and are quite coordinated you can also do the two external top fin fillets as well at this time). Do this three times and you done with the most unbelievable rocket strength, no injection drill holes needed. If you have an extreme minimum diameter with little to no space where it is near impossible to get a dowel in between the inside of the airframe and outside of the motor tube one technique that is used with great success is again go in from the bottom end and just to pour a little into the area you want to make an internal fillet and tilt the rocket tube to get it to flow where you need it to be, if you use about 15 to 30 minutes after mixing the Rocketpoxy will still flow when the tube is tilted but will still be stiff enough to stay put once tube is level again.

The Rocketpoxy was designed to give you about three viscosities to work within a single epoxy product. From the time you first mix it for about the first 0 to 20 minutes it will flow freely and self level pretty well and can be poured directly from a cup or even put into a syringe with a decent size dispense spout and injected quite well, especially if you have a larger size opening (1/8”diameter or more) or very easy if using a pneumatic dispenser, with this device a piston is forced by air pressure down into the syringe to push out the product, you can easily do very thick materials this way. If you want it to be a little flowable but really with no sags or runs but still trowels well and smooth (such as when doing a fin fillet) use it from about 30 to 45 minutes later after mixing, if you want to use it like a very thick epoxy putty wait after about 45 to 60 minutes and dip you fingers in isopropyl alcohol and you can pinch out an amount and roll into a ball or any shape and press into threads or a hole to repair. You can use up to 1 hour after mixing. Keep in mind the above is approximately and your exact conditions may need to be adjusted slightly, the temperature the epoxy is mixed at and the amount mixed will affect this (will cure faster in a 90 Degree F shop then a 60 degree F shop and will cure faster if mixing a very larger amount).
 
Some quick facts: West system epoxy is basically an unfilled epoxy made as a laminating epoxy I am not knocking it but just want you and others that may read this to know some of the differences of the epoxies being talked about on this thread. West system would be good for laminating fiberglass cloth to a plywood fin or fiberglass cloth to a cardboard tube or something similar. That is why West system epoxies have a thin viscosity of less than 1,000 centipoise, so the epoxy can flow around and into the fiberglass cloth, that is why they are called laminating epoxies. If you are purchasing a kit that has a filament would tube with G10 fins this would not be near as good a choice as using a filled high strength epoxy because you are not laminating the fiberglass cloth or fiberglass thread into a tube, the filament wound tube and G10 sheet manufacturer has already done this for you, you are at this point needing a high strength structural filled epoxy to assembly the tube and G10 fin and components for the best results. To use an unfilled epoxy similar to West System you would first need to add lots of fillers and thickening agents to make it anything even close for bonding strength to a filled high strength structural epoxy such as Rocketpoxy G5000. Rocketpoxy G5000 is in an epoxy classification called filled high strength structural epoxies. That is why Rocketpoxy has a viscosity of 350,000+ centipoise (basically a paste). If you love to formulate epoxies and have the time and knowledge to purchase, add, and mix the correct amounts and types of fillers needed to make a superior high strength filled epoxy then the west system unfilled epoxy is for you, if you want a high strength structural filled epoxy to use right out of the jar where an expert chemist with lots of test data has already done all the hard work for you by adding and mixing in the exact formulation of fillers then use something like the Rocketpoxy G5000. That is why filled epoxies do have better physical properties than unfilled epoxies for structural assemblies such as filament and carbon fiber wound tubes, G10 and carbon fiber sheet products, metals, plastics and generally any composites. Most serious composite assemblers that wish to use an epoxy ready to build with will prefer them, also that is why filled epoxies may cost a little more than an unfilled epoxy.

Also when comparing Rocketpoxy to an epoxy such as the Bob Smiths 20 or 30 minute epoxy or Proline 4500 the Bob Smith and Proline would be more brittle and have less bonding strength with some composite materials especially plastics and aluminum. Yes one of the major tradeoffs of a higher temp and faster setting epoxy is that they generally become more brittle. We feel that Rocketpoxy G5000 is the perfect tradeoffs of high strength, not brittle, curing time, and a decent temperature rating that is good for about 98% of all high powered flights. You may have a little more of a learning curve with the Rocketpoxy and may have to be a little patient about the curing times but it will give you a great finished product where lots of fin flexing and even some hard landings will not crack the epoxy. I’ll give you a quick test you can do put about ¼ - 3/8 inch layer of your West system, Proline , and Rocketpoxy G5000 epoxy into a plastic cup and let it harden, then whack all your samples with a hammer, you will notice quickly which epoxy did not shatter like brittle glass ( hint the RP G5000 will not easily shatter) , you will also notice which epoxy bonding strength stuck to the plastic cup and which epoxies ere easily popped off (you will not be able to separate the Rocketpoxy G5000 from the cup). You can have a higher temp and faster setting epoxy but if it is very brittle and less bonding strength this may cause failure well before any temperature comes into play. I’m not trying to get all the West systems, Bob smith and Proline users panty’s in a bunch, all these epoxies have their place and I sure as well know how this forum can get at times and not looking into getting into a big back and forth with this. If it works great for you more power to you, I’m just trying to answer a few questions I have been asked about this. Since my company is the Manufacturer of Rocketpoxy I do feel it is important to get the facts straight from the manufacturer who has actually done a lot of lab work and test data on this.

I'm aware of west systems use as a laminating epoxy. Currently, that's what I use it for. I just asked since I had heard of people putting additives in it and using it as a structural epoxy.

However thank you for the reply. When I start flying M and M motors I will likely switch back to Rocketpoxy to get most of the structural work done.

That being said, I have had one failure with both proline 4500, and G5000 in the past. The G5000 failure came from accidentally dropping a ~4lbs fin can on asphalt from about 4'. A through the wall fin popped off.

My proline 4500 failure came from a 38mm MD rocket that popped a fin off on landing. Parachute didn't deploy and the rocket fell from 4,200'.

I like the proline for its ease of use, and it was ultimately used in the rocket that was the subject of the OP, along with doing a full T2T with carbon and west systems. But I think my next rocket may use G5000 so I can be re-acquainted with it.
 
Quick comment I have on questions I have been asked on injection internal fillets, I personally don't like drilling injection holes in my FW tubes (making stress concentration holes right next to the slots doesn't do it for me) plus it wastes a lot of time to drill these, also trying to make sure everything inside is "leak proof" you know where every part has to be butted up together so no epoxy leaking though the motor centering rings can occur is difficult, tedious and time consuming and do not find any advantages to using the “injection" method, also I like to be able to see the internal fillets when finished, with injection method using black or colored FW tube this is not possible. I think this was originally developed because it was basically the only way you could do it when using the thin runny epoxies, I find it much easier to simply use a 1/8” to ¼” diameter wooden dowel to build my internal fin fillets laying the rocket horizontal and going in through the bottom and attaching the motor mount rear ring on afterwards, really this is pretty fast once you get the hang of it, you simply just dip the end of the dowel into the mixed Rocketpoxy twirl it a little to build up on the dowel, then go in the aft side of the rocket and twirl the Rocketpoxy into the fillet locations, you can see the internal fillets when finished and make them exactly how thick you want them, no guessing. I have even done minimum diameter rockets this way as well. I do four fillets per 120 degree rotation (for a 3 fin rocket), the two internal fillets on the bottom single fin, and the top two internal fillets on the two top fins per rotation, (or if you really want to speed things up and are quite coordinated you can also do the two external top fin fillets as well at this time). Do this three times and you done with the most unbelievable rocket strength, no injection drill holes needed. If you have an extreme minimum diameter with little to no space where it is near impossible to get a dowel in between the inside of the airframe and outside of the motor tube one technique that is used with great success is again go in from the bottom end and just to pour a little into the area you want to make an internal fillet and tilt the rocket tube to get it to flow where you need it to be, if you use about 15 to 30 minutes after mixing the Rocketpoxy will still flow when the tube is tilted but will still be stiff enough to stay put once tube is level again.

The Rocketpoxy was designed to give you about three viscosities to work within a single epoxy product. From the time you first mix it for about the first 0 to 20 minutes it will flow freely and self level pretty well and can be poured directly from a cup or even put into a syringe with a decent size dispense spout and injected quite well, especially if you have a larger size opening (1/8”diameter or more) or very easy if using a pneumatic dispenser, with this device a piston is forced by air pressure down into the syringe to push out the product, you can easily do very thick materials this way. If you want it to be a little flowable but really with no sags or runs but still trowels well and smooth (such as when doing a fin fillet) use it from about 30 to 45 minutes later after mixing, if you want to use it like a very thick epoxy putty wait after about 45 to 60 minutes and dip you fingers in isopropyl alcohol and you can pinch out an amount and roll into a ball or any shape and press into threads or a hole to repair. You can use up to 1 hour after mixing. Keep in mind the above is approximately and your exact conditions may need to be adjusted slightly, the temperature the epoxy is mixed at and the amount mixed will affect this (will cure faster in a 90 Degree F shop then a 60 degree F shop and will cure faster if mixing a very larger amount).

That's some great advise. And while I would like to do it that way, a split fin model with four centering rings makes that difficult to accomplish. Also the half inch of space between the motor mount and body tube (4" rocket with 75mm mount), doesn't help either. But as mentioned previously mentioned the task is now done and I think I have the strongest fin can I've ever made, by far.
 
For fillets on TTW rockets I normally use AeroEpoxy light filler. I have done this with 4" diameter rockets flying on 5G 76mm motors without any problems. I use JB to attach the fin to the motor mount and the Aeropoxy LIght for the fillet. Depending on the rocket I may do a layer of tip to tip FG. I originally used JB for the fillets on my rockets but after switching to Aeropoxy light and having not structurual issues I really like how easily I can sand the epoxy to the needed shape.
 
406 colloidal silica. [cabosil] You can usually find this at West Marine stores in every major city/mall or hobby shop under the "house" brand of filler. It's just fumed silica, light as feathers, but adds bonding strength to fillets. I know you what you stated about ordering , but maybe you do have it for other hobbies or drive right by a West or fiberglass supply type store [boats or car guys use it.]

I use it all the time, in various epoxies. Really cheap [20 bucks for big container] and handy to have around for all it's properties.

+1. Just picked up a new bucket for $16 at Woodcraft.

124528.jpg
 
I'm aware of west systems use as a laminating epoxy. Currently, that's what I use it for. I just asked since I had heard of people putting additives in it and using it as a structural epoxy.

However thank you for the reply. When I start flying M and M motors I will likely switch back to Rocketpoxy to get most of the structural work done.

That being said, I have had one failure with both proline 4500, and G5000 in the past. The G5000 failure came from accidentally dropping a ~4lbs fin can on asphalt from about 4'. A through the wall fin popped off.

My proline 4500 failure came from a 38mm MD rocket that popped a fin off on landing. Parachute didn't deploy and the rocket fell from 4,200'.

I like the proline for its ease of use, and it was ultimately used in the rocket that was the subject of the OP, along with doing a full T2T with carbon and west systems. But I think my next rocket may use G5000 so I can be re-acquainted with it.

I am wondering what kind of prep work you are doing before you used the RP G5000. The instructions are to use 60/80 grit sandpaper to really rough up all the bonding areas, inside the tube where your internal fillets with be, outside the tube where you external fillets will be, and of course the area of the fin that will be bonded, you can’t take any shortcuts with this is really takes a lot of elbow grease and sweat and needs to be done correctly. This greatly increases the surface areas that hold the bonds. I use an electric palm sander with 60 grit sandpaper which works great on the areas that can be reached with this, the inside tubes areas are difficult to get at but you can use a thick dowel with the 60 grit paper attached to one end on places that can’t be reached. I always make sure all areas to be bonding are cleaned with something like isopropyl alcohol, this will with clean off any mold release or grease that may be on the tubes. If you do all this I doubt the fin would have come off, I have had some large rockets come in from 5,000+ feet where the chutes may not have deployed all the way ( and /or only the drogue) with very rough landings with little to no damage using Rocketpoxy.
 
That's some great advise. And while I would like to do it that way, a split fin model with four centering rings makes that difficult to accomplish. Also the half inch of space between the motor mount and body tube (4" rocket with 75mm mount), doesn't help either. But as mentioned previously mentioned the task is now done and I think I have the strongest fin can I've ever made, by far.

Why would you need four centering rings, seems like overkill and a lot of added weight. I would just use top and bottom centering rings, 2 centering rings with thru the wall wall fins with internal and external fillets with RP 5000 would be crazy strong. And yes I have down split model fins this way. The four centering rings are probably recommended in the instructions or included in the kit not necessarily because of strength but because they would be needed for a dam to hold the thin runny epoxy in place with the injection method. If you are using a thicker epoxy such as Rocketpoxy and going in from the aft end to make your internal fillets the two center rings would work fine because the epoxy would stay in place and make nice thick internal fillets. I'm not knocking your build method at all just giving some alternative build method ideas here.
 
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I am wondering what kind of prep work you are doing before you used the RP G5000. The instructions are to use 60/80 grit sandpaper to really rough up all the bonding areas, inside the tube where your internal fillets with be, outside the tube where you external fillets will be, and of course the area of the fin that will be bonded, you can’t take any shortcuts with this is really takes a lot of elbow grease and sweat and needs to be done correctly. This greatly increases the surface areas that hold the bonds. I use an electric palm sander with 60 grit sandpaper which works great on the areas that can be reached with this, the inside tubes areas are difficult to get at but you can use a thick dowel with the 60 grit paper attached to one end on places that can’t be reached. I always make sure all areas to be bonding are cleaned with something like isopropyl alcohol, this will with clean off any mold release or grease that may be on the tubes. If you do all this I doubt the fin would have come off, I have had some large rockets come in from 5,000+ feet where the chutes may not have deployed all the way ( and /or only the drogue) with very rough landings with little to no damage using Rocketpoxy.

To build the rocket that I broke the fin off, I followed the instruction to the letter (Apogee' Level-2 by Madcow). That was over a year ago and I don't quite remember what I did for prep work, other than it included a lot of sanding.
 
Why would you need four centering rings, seems like overkill and a lot of added weight. I would just use top and bottom centering rings, 2 centering rings with thru the wall wall fins with internal and external fillets with RP 5000 would be crazy strong. And yes I have down split model fins this way. The four centering rings are probably recommended in the instructions or included in the kit not necessarily because of strength but because they would be needed for a dam to hold the thin runny epoxy in place with the injection method. If you are using a thicker epoxy such as Rocketpoxy and going in from the aft end to make your internal fillets the two center rings would work fine because the epoxy would stay in place and make nice thick internal fillets. I'm not knocking your build method at all just giving some alternative build method ideas here.

Four centering rings are used because it's a split fin model. I like to sandwich my fins between centering rings. Plus, a 4" to 3" G10 centering ring only weighs a few ounces, and weight is not a concern for this model. Also four centering rings were not included with the kit, only three are, I just like having that fourth one.

While I agree Rocketpoxy does stay in place quite nicely, t just wasn't my choice this time around. Pulling the internal fillets by hand is something I've wanted to do for a while, just haven't had enough clearance in between motor mount and body tube to get it done recently (4-3", 3"-54mm, ect.)
 
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