View Full Version : Chemistry Question?

08-20-2008, 08:14 AM
I know I've read this somewhere, and I'm certain someone on this forum can answer, Why won't Polyester cure over Epoxy? All I need is the short answer. I wouldn't understand the long answer. I've created myself about a week of work because of this situation.

James McMullen
08-20-2008, 09:24 AM
Polyester resin often needs a barrier film to cure hard--that's why the top coat of resin often has a surfacing agent such as wax added to it. Did you use a laminating resin or a surfacing resin here? Perhaps that's your problem? There might also be some sort of incompatibility with the amine blushing that epoxy is often subject to. There also might be the problem of insufficient, old or improperly mixed catalyst.

The real problem though is that polyester resin just doesn't stick as well to anything as epoxy resin does. Polyester resin does its best job of sticking even to itself when you lay everything up all at one time. I have had more success with long-term repairs of FG by using epoxy resin even if the original layup was of polyester or vinylester.

08-20-2008, 10:19 AM
Really short answer = don't use polyester for boats/marine purposes.

08-20-2008, 10:27 AM
The real problem though is that polyester resin just doesn't stick as well to anything as epoxy resin does.

That's the answer. Polyester resin has very poor adhesive properties.

Todd Bradshaw
08-20-2008, 10:37 AM
Not a very good short answer..... Polyester resin can work extremely well for boats and marine purposes...it just is not a very good choice for the type of coating/bonding/sealing/sheathing jobs commonly done with resin on wooden boats. I don't know of anything that would prevent polyester from hardening properly over epoxy, assuming that you understand its air-inhibited nature and take the proper steps to seal the surface. However, the main problem will always be that it doesn't stick or bond well to the cured epoxy surface and will likely delaminate in time.

08-20-2008, 11:55 AM
Thanks gentlemen. I am convinced of the superiority of Epoxy over Polyester. The situation I've got calls for heat tolerence of about 350 F and the Epoxy won't take it without softening. It's a vacume forming plug. I've got most of the epoxy off, and will now go back with polyester.
Thanks again.

Dave Carnell
08-21-2008, 08:29 AM
Polyester just doesn't stick. If the catalyst kicks off and doesn't cure you don't get another chance. Epoxy will eventually cure. The old text by Alan Vaitses, Covering Wooden Boats With Fiberglass strongly makes the point that mechanical fasteners are required to secure polyester-glass to a wooden boat.

08-21-2008, 09:11 AM
Polyester will soften the cured epoxy. I've seen this in surfboard repair. Always, epoxy over cured polyester. Never, polyester over cured epoxy.