Questions for the gods of epoxy and fiberglass

[DeltaVee]

I've recently come back to my first fiberglass project and some initial experience with the glue I've chosen sort of spooked me... I have a lot of uncertainty since as stated this is my first fiberglass project.

Some time ago I read a great discussion (it had some sort of title like "what the gov. doesn't want you to know about working with fiberglass". I spent a while trying to find it but so far no luck so I am taking a bit of a shortcut here. Some time ago I obtained a kit for a full-scale HV-ARCAS, and read through the aforementioned posting with great interest. The author (whose name escapes me) seems to know all there is to know about the subject of epoxy and fiberglass. One main takeaway from that article was the statement that one should glue whatever you just finished sanding... eg. if you sanded the interior of the business end of the airframe, you should within a short time, attach the motor mount. Something about the molecular adhesion being at its height shortly after sanding is completed... Assuming I'm recalling correctly of course.

So I went ahead and sanded every place that would touch paint or epoxy with 220 sandpaper...there were no glossy areas remaining. I then epoxied the forward centering rings to the motor tube (west systems 105 with 206 slow cure hardener). One thing I discovered was that my choice of epoxy seems incredibly thin. I decided on west systems because of all the rave reviews people in this forum have written not expecting it to be near water thin! I had to tack glue the centering rings to the motor tubes with medium viscosity cyano, and I then filleted with the west systems. It was amazing how much bled through what little gap between the ring and the tube existed.

So would anyone in their right mind use a filler for the motor mount centering ring fillets? I wouldn't think so but... Should I switch to one of the quicker curing hardeners?

Of course after all of that fun with the epoxy I let the project sit for better part of 18 months.

Since it's sat for so long do I need to sand the areas which will see glue again?


I'm expecting to have more questions coming up...

 

[watheyak]

Aeropoxy iss great stuff, but in it's unadulterated form it's a laminating resin, so it's thin to be able to easily wet out fiberglass or carbon fiber cloth.

The good news is you can turn it into a really good structural epoxy by adding some thickeners. I usually add equal parts colloidal silica and cotton flox until it's as thick as I want.

You can usually buy these two additives locally at West Marine, but they can be found cheaper online.

And sanding the shiny bits where glue will go is never a bad idea.

 

[DeltaVee]

I've got a container of west's 405 filler for non load bearing fillets... but my concern is strength of this stuff for use on the motor mount.

 

[watheyak]

The 405 is equivalent to micro balloons and is great for lightweight, cosmetic fairings. Not so hot for structural stuff. It's easily sandable so it's good for a top layer on fin fillets if you want to get really fancy.

 

[watheyak]

406 and 403 are the colloidal silica and the flox, respectively.

 

[cls]

I never use 206 slow hardner! 205 is the right stuff, in most places. Works best in moderate temperatures, like low 60s to mid 80s.

 

[DeltaVee]

I never use 206 slow hardner! 205 is the right stuff, in most places. Works best in moderate temperatures, like low 60s to mid 80s.

Thanks... I assumed slow cure would be good... Took a guess... with 205 do you get minimal bleed thru?

 

[DeltaVee]

406 and 403 are the colloidal silica and the flox, respectively.

Looks like a trip to west marine is in order!

 

[watheyak]

Oh oops, I see now that it's West Systems. Same info applies. I agree with CLS about the 205.

 

[Rschub]

As you discovered, laminating epoxy is usually too thin to be used as a good structural adhesive on fiberglass without adding modifiers.
West system is good for wood and cardboard since it was originally developed for use with that material and "soaks in". I build my LOC rockets with West and fast 205 hardener.
For fiberglass/carbon to fiberglass/carbon, structural adhesive is what you want, Hysol is the usual brand used, 9460 or 9430.
Rocket Poxy is mentioned here a lot. There are many options. This is where you get into the "Glue Wars".
Structural adhesive contains modifiers such as powdered rubber that toughen the adhesive to resist cracking. In composite joint design generally you want the panel or laminate to carry the loads, and the adhesive is there just to keep everything aligned.

Of course, depending on the loads obviously you can get away with just a butt joint fin with an epoxy fillet, but that is not good practice generally speaking in most other applications.

You should sand immediately before assembling fiberglass/epoxy composites, because it puts the surface in a high energy state that allows for a better chemical bond. There are machines available that electrically charge the surface to produce this effect, for the big wallet types.

 

[DeltaVee]

As you discovered, laminating epoxy is usually too thin to be used as a good structural adhesive on fiberglass without adding modifiers.
West system is good for wood and cardboard since it was originally developed for use with that material and "soaks in". I build my LOC rockets with West and fast 205 hardener.
For fiberglass/carbon to fiberglass/carbon, structural adhesive is what you want, Hysol is the usual brand used, 9460 or 9430.
Rocket Poxy is mentioned here a lot. There are many options. This is where you get into the "Glue Wars".
Structural adhesive contains modifiers such as powdered rubber that toughen the adhesive to resist cracking. In composite joint design generally you want the panel or laminate to carry the loads, and the adhesive is there just to keep everything aligned.

Of course, depending on the loads obviously you can get away with just a butt joint fin with an epoxy fillet, but that is not good practice generally speaking in most other applications.

You should sand immediately before assembling fiberglass/epoxy composites, because it puts the surface in a high energy state that allows for a better chemical bond. There are machines available that electrically charge the surface to produce this effect, for the big wallet types.

That last paragraph rings a bell from the other post I was mentioning. Points well taken... Thanks!

 

[Rschub]

Here is the thread you were referencing BTW:
https://www.rocketryforum.com/threa...our-government-doesnt-want-you-to-know.58389/

 

[cls]

Thanks... I assumed slow cure would be good... Took a guess... with 205 do you get minimal bleed thru?

205 + resin is the same viscosity.

Save the 206 for hot days.

one time I was repairing some cracks in a hot tub. On a hot day, hot surface (perhaps over 150F!), in the sun. Mixed up the 205 and resin, tried to work it in the cracks, suddenly the pot got too hot to hold, I set it on the tub surface, in seconds it gelled and started smoking! I bought a new hot tub instead.

 

[Steve Shannon]

I use both Aeropoxy or West Systems laminating resin and then I add silica or pulp to thicken it when I need it. I have both fast and slow hardeners for the West Systems. I for centering rings and fillets I would use thickened and fast hardener. For laminating I would use slow hardener. I paint the surfaces to be joined with unthickened epoxy first and then fill the joint or fillet with the thickened epoxy.

 

[John Kemker]

Aeropoxy iss great stuff, but in it's unadulterated form it's a laminating resin, so it's thin to be able to easily wet out fiberglass or carbon fiber cloth.

The good news is you can turn it into a really good structural epoxy by adding some thickeners. I usually add equal parts colloidal silica and cotton flox until it's as thick as I want.

You can usually buy these two additives locally at West Marine, but they can be found cheaper online.

And sanding the shiny spits where glue will gonis never a bad idea.

Aeropoxy PR2032 is laminating. ES6209 is thicker and structural.

 

[DeltaVee]

I use both Aeropoxy or West Systems laminating resin and then I add silica or pulp to thicken it when I need it. I have both fast and slow hardeners for the West Systems. I for centering rings and fillets I would use thickened and fast hardener. For laminating I would use slow hardener. I paint the surfaces to be joined with unthickened epoxy first and then fill the joint or fillet with the thickened epoxy.

Ok, so at this point I'm thinking I should get West systems 205 (fast) hardener

Now regarding the actual placing of my motor mount, I have some choices to consider... While I'm pretty sure I could just go ahead and epoxy the motor mount in without fillers, I'd have to be mindful of the potential for excessive sag/bleed thru between the rings and tube wall. That suggests I ought to use a filler in the epoxy for the motor mount (Correct me if this is a bad idea!).


From the west systems chart regarding fillers, the rating is 4/4 stars (excellent), 3/4 stars (very good), 2/4 stars (good), 1/4 (fair).
The chart is found here: https://www.westsystem.com/filler-selection-guide-2/
I'd love to know if anyone knows the diff between "Bonding Hardware...

The candidates for fillers from "highest strength" to "easiest sanding" are :
a. the 404 filler has 4/4 stars (excellent) for bonding hardware and 3/4 stars for general bonding
b. the 406 filler has 4 stars for "bonding with fillets" and three stars for all other types of bonding (general. hardware etc)
c. the 403 filler has 4/4 stars for laminating, while 2/3 for bonding with fillets and 3/4 stars for General bonding and hardware bonding
d. the 405 filler (which I have currently) has 3/4 stars for "bonding with fillets" which is a "very good" rating, but 2/4 for all other types.
this choice seems better for fin fillets... (oddly, 406 has a 4/4 star usage rating but not as easy to sand apparently)

For the motor mount, what type of bonding would this be? It seems to me I'd want to use (if permissible) the 406 since none of this will be sanded down later! Internal fillets would also probably be well handled by these choices since, again, no sanding is required. External filleting seems best done with 405 since internal fillets will carry some of the load so maximum strength would not be necessary. So it would seem that 406 might fight my needs... @watheyak - Is your reasoning to mix 406 and 403 to get better sanding behavior.

​

 

[OverTheTop]

I only use West Systems 105/206. I guess I am used to the viscosity.

 

[Rschub]

The 205 hardener will cure pretty quick, I wanted to seal the wood of my bulkhead so I brushed the epoxy from the center out, after a few minutes it will start to stiffen up and you can then do the cracks with minimal bleed through, which no one will ever see anyway.
If you want to use filler due to the all fiberglass construction all but the last 2 will be fine. 406 does have the highest average performance.

The lightweight fillers are used to ease the finishing process, in general terms

Note that they call for either Vaseline or peanut butter consistency, this is left to your judgement on how much filler to add. Just add a little at a time.

 

[OverTheTop]

I usually go for toothpaste consistency when adding microballoons.

 

[Rschub]

Microballoons should only be used in finishing filler and never for structural bonds.

 

[SolarYellow]

Download the West Systems Epoxy User Manual & Product Guide here:

https://www.westsystem.com/instruction/user-manual-product-guide/

 

[DeltaVee]

Trip to west marine today... Picked up the 406 colloidal silica filler and 205 hardener. Now all I need to do is set up access to a drill press!

 

[David Schwantz]

All you guys and your fancy fillers I like to use chopped up 6 oz fiberglass cloth. Just cut up a bunch so the strands are about 1/4" long, mix into already mixed epoxy, of any kind. And apply. You will never find anything harder than this. Can grind it, drill it, tap it. My first time doing it was on an RC plane, Yellow P-47. Had the plane over 20 years and was still going strong when I finally sold her.

 

[Rschub]

You will never find anything harder than this

You won't find too much heavier either.

Thousands of boats have been made with that stuff, so it has it's uses.

 

[David Schwantz]

All depends on your skill of application.

 

[DeltaVee]

Ok so now to transition a bit from epoxy and fillers (I think I've got a plan now for that)... as far as a project that uses 4.5" diameter filament wound fiberglass and set up with a dual-deploy e-bay etc, there are a few mechanical questions I've got now...

1) when going to drill the ports for the altimeter in the switch ring, what "best practices" are there for ensuring near-perfect symmetry of the holes? I.e. what's the closest you can get to perfect 90 degree separation? I'd like it to be at least to within a few hundredths of an inch.

2) Involves use of push pins in the e-bay/payload portion... I've got a pile of 1/8" push pins. If I were to use these would 4 be enough? Would something like screws be recommended instead?

3) Similar to 2) I've got fairly small shear pins... I can get away with two on my 3.1" cardboard Cherokee G... can I use these if I
up the number to 4? Would 2 be enough?

Thanks for merciful responses!

 

[John Kemker]

Ok so now to transition a bit from epoxy and fillers (I think I've got a plan now for that)... as far as a project that uses 4.5" diameter filament wound fiberglass and set up with a dual-deploy e-bay etc, there are a few mechanical questions I've got now...

1) when going to drill the ports for the altimeter in the switch ring, what "best practices" are there for ensuring near-perfect symmetry of the holes? I.e. what's the closest you can get to perfect 90 degree separation? I'd like it to be at least to within a few hundredths of an inch.

2) Involves use of push pins in the e-bay/payload portion... I've got a pile of 1/8" push pins. If I were to use these would 4 be enough? Would something like screws be recommended instead?

3) Similar to 2) I've got fairly small shear pins... I can get away with two on my 3.1" cardboard Cherokee G... can I use these if I
up the number to 4? Would 2 be enough?

Thanks for merciful responses!

1) One port should be sufficient. No need to make symmetrical or get it to such precision. If you really insist on two opposing, symmetrical, holes, then take a piece of paper just a little less wide than the switch ring. Wrap it all the way around the switch ring and make a line where they overlap. Now, fold that from the other end to the line and mark the fold. Wrap it back around your switch ring and use an awl or center punch to mark the switch ring on both lines. Drill where you marked. No need for measuring.

2) What do you mean by "push pins?" Picture?

3) What's the purpose of the "push pins?" If you're meaning "shear pins" then you can get away with just one, to hold things together until ejection. Too many, and you risk preventing separation when you want it to happen. I've used a single toothpick on a 4" diameter fiberglass rocket. Fiberglass sheared that toothpick right off, but it held together until it wasn't supposed to.

 

[DeltaVee]

The items on the left are "push pins". When engaged, the end flares and lock in place. If use these to hold the forward tube to the electronics bay, then it allows for disassembly with relative ease. On the right are a couple of shear pin screws. They're mad of nylon... BUT for sure a couple of toothpicks are easier to come by! Now why didn't I....

 

[DeltaVee]

Here's another one...

Is it better to remove the metal tip on the nose cone when finishing/painting?
Does it matter?

 

[troj]

Aeropoxy iss great stuff, but in it's unadulterated form it's a laminating resin, so it's thin to be able to easily wet out fiberglass or carbon fiber cloth.

AeroPoxy makes both a laminating resin and an adhesive resin. The latter is significantly thicker.