Page 2 of 2

Re: Epoxy vs. Polyester for Fiberglass Repair

PostPosted: Mon Apr 01, 2019 5:54 pm
by Esprit2
> This is pretty much in line with my thinking. Poly cures in 15 minutes, Epoxy
> more like 15 hours. Do you want to wait 60 times as long for your repair to set?

I disagree. That may be how you choose to apply those resins, but it's not an accurate description of them.

Polyester is a one-part resin that requires a catalyst to promote it's cure. Given time, like years, it will eventually cure on it's own. But if you want it to cure today, then you need to add the catalyst. If you want a 'fast' cure, like in the next half hour, or in 15 minutes, then you add progressively more and more catalyst. The more catalyst you add, the faster it cures, and the faster it cures the hotter it gets. The downside is that forcing a faster cure gives up some physical properties... ie, the hot-cure resin won't be as strong once cured.

If the mixed resin gets too hot for you to handle the mixing cup with bare hands, or hot "enough to melt other plastics near it", then that batch has been grossly over-catalyzed to the detriment of the final product. "Warm" is all that is required... or desired. A hotter batch than that is more about the user's impatience than it is about an optimal cure.

"Working Life", or "Pot Life" is the time from when the liquid resin is mixed until it begins to 'gel'. It's not wise to attempt to work gelled resin into fiberglass, it won't fully saturate the fibers.

"Green Cure" is when the resin gets hard. Or at least stiff, if not fully hard.

"Full Cure" is when the the vast majority of the molecular cross-linking is complete. It will take a very long time for a 100% cure, but something in the 95-98% cure range is considered "Full Cure". You can remove the part from the mold, and/or proceed with finishing and painting.

In general, a working life (ie, pot life) of 30 minutes is about right for a mixed batch of polyester. For larger lay-ups, reduce the amount of catalyst to allow more working time so you can complete the lay-up before the resin gels-up.

For the resins, and for the catalyst ratios that are desireable for laying up fiberglass parts or repairs, a green-cure in an hour+ is about right. But it's best to allow 24 hours before de-molding a new part, or proceeding with finishing & painting.

Epoxy resins don't all take 16 hours to cure. Such a statement is like comparing Full Cure to Pot Life. True, you can get a very slow cure epoxy if you desire, but that would be a choice you made. You can also buy an epoxy that cures in 3 minutes without excessive heat. To say polyesters cure in 15 minutes and epoxies cure in 16 hours is presenting a very biased scenario. While 'possible', it's rarely true.

Time should not be the driving factor, take the time to do the job right. There's an old saying... "There's never time to do it right, but there's always time to do it over".

> You can also roll epoxy; don't even think about trying that with the short pot life of poly.

You can and should roll both resins as a means of working all air bubbles out of the layup, and ensuring full, intimate contact with the mold surface, or with the existing surface surrounding the repair. If polyester doesn't allow sufficient time for rolling, that's only due to the user's 'choice' to mix a hot batch that cures too quickly.

Fiberglass matte contains a 'sizing' that holds the random, loose fibers together in a sheet prior to application.

Polyester resin contains strong solvents (acetone, ketone, etc) that quickly break-down the sizing, allowing the matte to quickly go limp, and easily conform to the surface of the mold or part being repaired.

In contrast, epoxy contains no solvents, and the matte initially remains stiff when wetted. But given a little time, the sizing eventually gives up, and the matte becomes as limp as it would with polyester. It just takes a little time... but not long.

> Getting the shell to a good surface after repairs will be easier with poly.

Both resins require a primer/surfacer as part of the finishing process, and it is that material that will have the greater impact on surface finish. Both resins will cure with a glossy surface that is not the best for paint adhesion. The raw 'glass' should be sanded/ flatted prior to priming in order to both "flatten' the surface, and to give it some 'tooth' for the primer/surfacer to grip.

Tim Engel

Re: Epoxy vs. Polyester for Fiberglass Repair

PostPosted: Fri Apr 19, 2019 2:33 am
by deadline
Chris, Bud, Dan (StressCraxx), Pete, Rich, Steve, Tim, Gordon, and all the rest,
Thank you all for your responses and all the input. There is a lot here for me to think about and study over.

I thought this would be a straightforward question. Use brand X, steer clear of Y, and fix mistakes with Z. I can see there are many factors to consider (weave, tint, working life, pot life, shrink rates, ?) and a lot more materials and options than I realized. My take away from all of this is there is a lot to learn and maybe this pile of S3 is not the best place for me to do that learning. As stated, I?m dealing with a bit more than just some spider cracks. For now it will just have to stay a pile.