Epoxy filet question
 

Can you use epoxy for a filet over a painted fin and still get good adhesion? I'm building a larger rocket with thru the wall fins and it would help to paint the fins first. Anyone have thoughts on this? Thanks.

Rick

I presume the reason you want to paint the fins first is that they are a different color than the body tube?? You are probably better off to mask the body tube and paint the fins after you have completed all construction. If you MUST do it the way you propose, use epoxy on the through-the-wall tabs (don't get any paint on the tabs) and, if possible, put an epoxy fillet on the tab/engine mount joints, dabbing the epoxy on using a long piece of dowel before installing the rear adapter of the engine mount. Also, lightly sand the fins near the root edge before applying your external epoxy fillet.

GHRocketman has a pretty strong chemistry background and could probably give you some extra info.

Joe

The problem with epoxy fillets AFTER paint is that the epoxy does not penetrate the paint down to the substrate; it adds virtually ZERO strength as the bond is not to the wood and tube. Most all Epoxy also dries transparent yellow, NOT clear, which is NOT something I'd want on top of my paint job. Masking is a little tougher with the rounded epoxy fillets if the fins are to be a color other than the tube; make the fillets very small with about a 1/16" radius and they will still add a LOT of strength without being too hard to mask. Fillets over the paint make no sense unless for aerodynamics; they will NOT add strength; just WEIGHT. When properly used, the epoxy fillets actually penetrate the wood and tube if one uses anything slower than 5-minute epoxy. I usually use 15 or 30-minute cure.
If you have through-the-wall fins, fillet the fins INTERNALLY only without putting the lower engine mount centering ring on until you fillet the fins.

Thanks for the help guys. With that said, should I sand off the Aero Gloss sanding sealer by the root edge of the fins to help the glue adhere better? I'm guessing yes. Since this is a larger rocket, I thought that it would be easier to do some of the finishing before gluing the fins on. I might have gotten ahead of myself.

Thanks,
Rick

Yes, Rick, you should sand the root edges just enough to remove any sanding sealer and expose fresh balsa. In the future, if you are going to pre-finish your fins before attaching to your rocket, it's a good idea to work some glue into the pores of the balsa on the root edge to exclude sanding sealer. It's also advisable to work glue into the fin surfaces that will eventually be under your final glue fillet (you can do this by masking all of the fin except about 1/8" along the root edges and then working glue into the exposed areas).

Joe