I’m almost embarrassed to ask this question, but here goes.

I am building a giant scale plane, of local design. I have beefed up the fuselage with triangle stock. The designer of the plane recommended fiberglassing the firewall area. So, I cut my piece of fiberglass. Test fit it with no glue. When satisfied, I mixed up a batch of BSI 30-minute epoxy. I stood the fuse up on the nose. Poured some of the epoxy on the fiberglass and started to spread the epoxy with a wooden tongue depressor. The epoxy would not penetrate the glass cloth. BTW - this is pre-packaged glass cloth. Hobbico or something. Medium weight - like you would use for a wing center section. Anyway, I saw what was happening so I grabbed my heat gun and applied heat. The epoxy turned to water as I expected and flowed into the weave. But then, the cloth was sliding everywhere. Any attempt to spread the epoxy with a tongue depressor or my fingers resulted in the glass cloth sticking to said tools and pulling away from the firewall. I wrestled with the whole mess for 5 minutes before I just yanked the cloth, threw it in the trash, and used the heat gun to spread the remaining epoxy to fuel-proof the area.

I need to try again. But need some tips. What am I doing wrong? In full disclosure, I am an old builder from way back before ARFs and the foam invasion. I love to build and feel like I am fairly proficient in most areas. But I have always hated using epoxy. For some reason, it has always come with some level of anxiety, be it cure time, clean-up, whatever.

Any tips you have are welcome. Again, I hate to even admit I struggled with something so trivial. But I can’t help but think I am doing something wrong.

Thank you.

Jason

You should actually use fiberglass resin but if I were gona use what your using I'd thin the epoxy with alcohol and paint on a coat to the wood First then quickly stick the cloth to that...then paint over the top.

I have always used z-poxy finishing resin as it is thinner and has a longer cure time. Still a pain in the a$$ to work with fiberglass, but this makes it easier. Also make sure the air temp is warm, warmer the better.
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One more tip I have is to first put the resin on the wood, and then lay the glass on. You will see the cloth get wet and that is all you need. It helps the glass to stick to the wood instead of your fingers/tongue depressor.

I just went throught the same thing with my P-47 on the cowl mounts.. i used 30 minute epoxy and the same kind of resin.. i applied my epoxy with a flux brush.. and would dab it as much as i could while it was curing with the end of the bristles.. if i was gonna do a bigger area i would thin out the epoxy as stated earlier and try to coat the firewall first to stick it in place and then dump the epoxy over top the resin and spread it with a small brush. And wear latex gloves!! It gets messy fast

Regular epoxy will not "wet out" the cloth you need to use either a polyester or epoxy resin designed for wetting out cloth. Like Kwilson said Z-poxy works great however polyester resin is a bit easier to work with IMO

Okay, thank you. I have BSI 20 minute finish-cure epoxy that I was going to use on the wing center section. Will that work for the firewall as well?

I'm not familiar with BSI but you want to make sure it says something like "finishing resin" or laminating resin or similar. If not sure let the seller know what you want to use it for.

BSI is Bob Smith Industries products. This is what I was referring to. I have had a number of people tell me this stuff is great for glassing wing center sections.

I guess I need to do some research on the difference between resin and epoxy.

Finishing Resin is either Epoxy or Polyester based, that's sometimes where the confusion begins, they are pretty much the same, and are for glassing and finish coats

But two part 5, 15, 20 or 30 minute epoxy is thicker than "finishing" resin because it's really designed for gluing parts together,, not glassing, it can be thinned with rubbing alcohol to make it penetrate glass cloth better, I've done it many times in a pinch

Personally I use west system epoxy for glass work

good luck

Does finishing resin have the same amount of strength as regular epoxy? To me finishing resin is for exactly that, finishing, where you intend on sanding and then painting a finish.
I have always used the BSI type stuff in 30 minute and even 60 minute "I think there was a 60 minute", for reinforcing firewalls and especially for wing center sections, way back when we used to build the airplanes we fly. Lol.
I would always apply epoxy first, spread it out, and then lay the fiberglass cloth on top and it would soak right through.Then squeagy out any excess and let it dry overnight.

I was going to ask the same thing. But after doing some reading, there doesn’t seem to be any difference with the exception of working / curing time. Maybe I’m wrong, though.

It has to do with the viscosity, as to how well it wets out cloth. You want a low viscosity. West systems works great. They have an easy pump system to make sure the mixing ratio is correct. If you have a Tap plastics nearby you could also pick up some marine epoxy. Medium to slow cure is helpful. Typically they are not one to one mix ratios. You will definitely be able to tell when the epoxy pot life ends as it will start to behave like the OP has indicated sticking to everything but what you want it to. Definitely want to be done wetting out and squeeging off the epoxy before that happens. Good luck to the OP, it is definitely rewarding once you get the hang of it. Suggest laying up some cloth on some scrap pieces of wood to get the hang of the pot life and wetting out before moving forward on the model.

Originally Posted by Motomazzo

I was going to ask the same thing. But after doing some reading, there doesn’t seem to be any difference with the exception of working / curing time. Maybe I’m wrong, though.

Well the commonly told truth was that the longer the cure time, the stronger the epoxy is,

Frankly I don't know if that's true or ole pit bench lore,, just use what works for the situation,, what ever cure time you pick is going to be plenty strong for a toy airplane application