Bondo Fillets

[Walldiver7]

I am considering using bondo for the exterior fillets on my latest hp rocket build. All interior fillets are Aeropoxy. In the past I have used Aeropoxy for the exterior fillets, but have loathed the sanding process needed for my personal satisfaction. I'd appreciate comments about whether or not this a sound practice. Thanks.

 

[dixontj93060]

Do you mean the real auto two-part Bondo (not Bondo glazing putty)?

 

[Walldiver7]

Do you mean the real auto two-part Bondo (not Bondo glazing putty)?

Yes, correct, the real auto two-part Bondo. I've used it as a filler in the past for my epoxy fillets that weren't perfect, but this time I thought I might do away with using epoxy on the external fillets and instead use entirely Bondo.

 

[dixontj93060]

Yes, correct, the real auto two-part Bondo. I've used it as a filler in the past for my epoxy fillets that weren't perfect, but this time I thought I might do away with using epoxy on the external fillets and instead use entirely Bondo.

What have epoxy have you been using? For the most part, epoxy (without additives) is self-leveling and will shape nice and smooth to fill any fillet.

 

[tomsteve]

using only bondo for fillets has a high chance of cracking if/when a fin tip hits the ground. unless theye going to be very thin.

 

[rharshberger]

Bondo is heavy, is brittle thick layers, has a very short pot life. I have tried it and like the results I get with epoxy better, for reasons of weight, epoxy with amendments sands easier, and if the fillets are laid down with a bit of practice they might even be nice enough to leave as is.

 

[Nathan]

I sometimes use Bondo putty to smooth out cosmetic imperfections in epoxy fillets but I don't think it would work very well to use Bondo to make the fillets. Bondy putty is not nearly as strong as epoxy so if the purpose of the fillet is to strengthen the fin joint then you should probably stick with epoxy.

If you want perfectly smooth fillets then you can build them in two stages; first make fillets using structural epoxy like West Sytems or Aeropoxy. Then after that hardens, cover the fillets with a thin layer of 30 minute Bob Smith epoxy, which self-levels very nicely. That's how I made the fillets on my Frenzy XL . . .

 

[GregGleason]

Have you tried adding microballoons to the epoxy? For non-structural epoxy fillets, they make the epoxy much easier to sand. After mixing the epoxy resin, you mix in the amendment (in this case, microballoons) to the desired ratio. It's kind of an art, so you may need to experiment until you get the ratio that works best for you.

https://www.amazon.com/dp/B0015H1FDI/?tag=skimlinks_replacement-20

Greg

 

[Binder Design]

I've done it. Works really well IMO. Best fillets I ever did were using Bondo. Lasted several years until I catoed the whole fin section.

 

[Onebadhawk]

I use a high quality epoxy for the fillets...
Then a 2 part body putty ( Bondo ) very thin over the epoxy,,
then a 2 part auto glazing compound very thin over the putty..

Flawless fillets....

Teddy

 

[Walldiver7]

I use a high quality epoxy for the fillets...
Then a 2 part body putty ( Bondo ) very thin over the epoxy,,
then a 2 part auto glazing compound very thin over the putty..

Flawless fillets....

Teddy

I have been doing fillets this way for years.... and the results have been great. Since I wasn't concerned about adding any more strength ...most of stress is applied to the MMT / fin root joint and my internal fillets are plenty strong anyway... I was just wonder if I could skip the external epoxy fillet and use bondo only. Bondo is just SO easy to shape. Thanks for your response.

 

[Walldiver7]

What have epoxy have you been using? For the most part, epoxy (without additives) is self-leveling and will shape nice and smooth to fill any fillet.

In the last ... 6 builds?, I've used the same structural epoxy (Aeropoxy 6209) for fillets as I've used during the builds. 6209 is not self leveling at any temp. lower than 70f.... so I've put the hairdryer to use to make if flow. In my experience, if you mix anything into it to add strength, leveling becomes even a bigger chore..... without cutting it with denatured alcohol. Aeropoxy 2032 is a laminating epoxy, and has a bit more viscosity than water. I haven't tried mixing in milled fg to see if would stiffen it to the point that I'd feel comfortable giving it a try for fillets .... and maybe that's where I should start instead of just going to bondo.

 

[Walldiver7]

I've done it. Works really well IMO. Best fillets I ever did were using Bondo. Lasted several years until I catoed the whole fin section.

Good to hear that the bondo only fillet was up to the task.

 

[rharshberger]

In the last ... 6 builds?, I've used the same structural epoxy (Aeropoxy 6209) for fillets as I've used during the builds. 6209 is not self leveling at any temp. lower than 70f.... so I've put the hairdryer to use to make if flow. In my experience, if you mix anything into it to add strength, leveling becomes even a bigger chore..... without cutting it with denatured alcohol. Aeropoxy 2032 is a laminating epoxy, and has a bit more viscosity than water. I haven't tried mixing in milled fg to see if would stiffen it to the point that I'd feel comfortable giving it a try for fillets .... and maybe that's where I should start instead of just going to bondo.

I use US Coposites 635 laminating resin with milled fiberglass and a little cabosil to make my external fillets. Usually to a no-run consistency, and pulled with a piece of pvc pipe the diameter I want, allow it to sit for a bit to gel even further and then smooth again with the same pvc lubed with denatured alcohol. No reason 2032 with similar additives wouldn't work and be stronger than Bondo even with all the additives.
I further touch up is needed the a bit of 635 with phenolic microballons is added and skimmed on.

 

[Walldiver7]

I use US Coposites 635 laminating resin with milled fiberglass and a little cabosil to make my external fillets. Usually to a no-run consistency, and pulled with a piece of pvc pipe the diameter I want, allow it to sit for a bit to gel even further and then smooth again with the same pvc lubed with denatured alcohol. No reason 2032 with similar additives wouldn't work and be stronger than Bondo even with all the additives.
I further touch up is needed the a bit of 635 with phenolic microballons is added and skimmed on.

Why cabosil? Is the cabosil just to help with thickening?

 

[dixontj93060]

In the last ... 6 builds?, I've used the same structural epoxy (Aeropoxy 6209) for fillets as I've used during the builds. 6209 is not self leveling at any temp. lower than 70f.... so I've put the hairdryer to use to make if flow. In my experience, if you mix anything into it to add strength, leveling becomes even a bigger chore..... without cutting it with denatured alcohol. Aeropoxy 2032 is a laminating epoxy, and has a bit more viscosity than water. I haven't tried mixing in milled fg to see if would stiffen it to the point that I'd feel comfortable giving it a try for fillets .... and maybe that's where I should start instead of just going to bondo.

Yep, have never used Aeropoxy 6209, just looked it up--50,000 cps viscosity reading is ridiculous. I would suggest using something else.

My choice for a structural epoxy is System Three T-88 with a more reasonable flow around 10,000 cps.

But for external fillets, heck, it is mostly for cosmetics and aerodynamics, 15 min BSI works well. Or try the 2-part Bondo, it may work.

 

[dixontj93060]

Why cabosil? Is the cabosil just to help with thickening?

If you are just seeking thickening and need sandability, go with the phenolic microspheres (microballoons in post #8) mentioned earlier. They thicken and are easier to sand than silica (Cabosil).

 

[rharshberger]

Why cabosil? Is the cabosil just to help with thickening?

Yes, it has no other purpose. The milled FG will thicken some but not enough.

 

[o1d_dude]

My structural bonds are all Aeropoxy ES6209. The fillets are done with West epoxy and amendments for ease of smoothing/sanding. I have some Aeropoxy Light but haven't tried it yet.

 

[3stoogesrocketry]

I've used kitty hair , fiberglass imbeded bondo , several times. I will put down masking tape to get the borders , both sides of the same fin . Then lay down a nice bead on both sides of the same fin. Then take yourself a piece of three-quarter inch wooden doll with a piece of 2 inch packing tape stuck to it , and press them into the bondo. I use small clamps to pull the dolls together creating a sandwich with the fin in the middle. let them sit for an hour then pop them right off. the tape will help remove extra material for cleanup . I suggest doing a test piece before attempting on your actual rocket. I found out once that if you use way too much , and it gets all over the rocket it can be a pain to clean up.

Eric

 

[amarillo_rocket]

I don't like sanding bondo it's a pain in my opinion. Aeropoxy light makes great fillets. It's the consistency of a thick toothpaste, goes on smooth and stays put. I put a little bit of water on my finger run it down the fillet to make it extremely smooth. A light sanding with 400 grit paper makes for a very smooth fillet. My second choice is laminating epoxy epoxy with microballoons added as a thickner. The microballoons make it easier to sand but it still is not as easy to sand as Aeropoxy light. I let the epoxy set for a bit before applying which keeps it from running. How long it sets will depend upon the epoxy. When I start to notice that it is thickening almost to a consistency that will stay in place I apply it. Waiting too long is obviously an issue.

 

[Walldiver7]

I don't like sanding bondo it's a pain in my opinion. Aeropoxy light makes great fillets. It's the consistency of a thick toothpaste, goes on smooth and stays put. I put a little bit of water on my finger run it down the fillet to make it extremely smooth. A light sanding with 400 grit paper makes for a very smooth fillet. My second choice is laminating epoxy epoxy with microballoons added as a thickner. The microballoons make it easier to sand but it still is not as easy to sand as Aeropoxy light. I let the epoxy set for a bit before applying which keeps it from running. How long it sets will depend upon the epoxy. When I start to notice that it is thickening almost to a consistency that will stay in place I apply it. Waiting too long is obviously an issue.

I haven't tried Aeropoxy light ... I will give that a try on the next one.

 

[neond7]

My structural bonds are all Aeropoxy ES6209. The fillets are done with West epoxy and amendments for ease of smoothing/sanding. I have some Aeropoxy Light but haven't tried it yet.

I've tried many methods but pretty much settled down to this formula. Aeropoxy 6209 structural for all internal bonding and bonding fins to the motor mount. And yes, its slow cure so I take three days to attach three fins.

For external fillets, I use West Systems - one pump each of epoxy and hardener, then I add one heaping spoon West Systems 404 high density filler and one heaping spoon 406 colloidal silica. Takes a lot of stirring to get it to a thick pancake batter consistency. I blue tape off my fillet lines, apply the epoxy mixture with the back of a spoon making a few slow passes to smooth it, shape it, and remove any extra. Then I pull tape. I use my finishing sander to smooth off the front and back edges, but very little sanding of the fillet itself. Then I use a thick Bondo application and lots of sanding. Then more Bondo, more sanding. Then primer and paint.

Yes its a slow, messy process, but it looks awesome and I've had zero issues with fillets cracking. My early work with 30 minute poured epoxy had several issues with the epoxy cracking. Epoxy is very brittle by itself without fillers.