polyester-epoxy resin compatibility?

John Robinson

John Robinson

Well-known member
Back in junior high school, high school and later I used to repair and build surfboards. I've been out of it a long time but back then we used two different types of polyester resin; laminating resin for the fiberglass cloth and finishing resin for the gloss coat. The laminating resin was too soft to sand and polish, it sort of smeared, sort of like trying to sand epoxy that hasn't hardened enough.

The process was to wet out the fiberglass cloth with a squeegy like we do, the poly resin seemed to wet out the class easier, there was no need to pre soak the cloth in resin. Anyway, you would squeegy out as much resin as possible to keep the weight of the board down. This left a prominent cloth weave showing after the resing hardened. Rather than sand the texture out for a smooth but weaker surface, we would keep the weave of the fiberglass cloth intact and fill the texture with finishing resin which was much harder. After this hardened we would sand, then polish the board with rubbing compound and wax.

My question relates to the finish coat on the hull. I read in the directions about sanding, then rolling on a finish coat of epoxy. Is the sanding just a light roughing of the surface so the resin sticks better? How does the epoxy do as a hard finish coat? Would there be any point in using a finish coat of polyester resin of the laminated epoxy? I'm worried about sanding through the weave of the cloth eliminating some of its strength.
 
Mostly the sanding on the initial coat will be to clean off any blush and to give a bit of "tooth" so the next coat will bite well. I used 220 on a block with a pad and scuffed..then wash off with a wet rag. If you get right after it and use non blush epoxy you simply recoat within 24 hours and get a chemical bond which is the best way to do it.
 
You should wash off the blush with hot water and a wash cloth, then sand lightly to get some grip for the next coat of epoxy, which you can thicken with some silica. This will do something like your finish coat on the surfboard; fill all the little gaps in the weave, then sand once the second coat is cured for a final finish. Depending on how nice of a finish you want, you can repeat this process over and over again until you've got it perfect. Some duck boat builders spend more time on this "fairing" than they do on the actual construction of the boat itself. I'm not one of them, as the ducks won't notice, and once it's good and muddy, your padnahs won't either.

Ed.
 
Laminating resin has no wax inhibitor, finishing resin does to allow the top coat to completely harden both the same stough finish has the wax added. If using polly. use all polly., if epoxe use epoxe!
 
Thanks for the replies.

It seems that I am using a non-blush epoxy as I don't see any blush and the epoxy is very hard and glossy. I bought my epoxy from US Composites and am very happy with the quality. I bought the slow hardening 2:1 system as it was 90-100 when I bought last summer. Now it's 15-40 degrees outside, but with a couple of heat lamps the epoxy hardenes well in 2-3 days in my 40 degree garage. I'm not pre-wetting the fiberglass in a tank as that seemed too messy, but we are making sure to saturate the cloth completely.

I think I'll do what Lee suggested, add a filling layer of epoxy within 24 hours of fiberglassing the bottom. We are still installing keel and bulkheads so it will be a while before we flip the boat and do the bottom, I was just thinking ahead. Last night we fitted all the bulkheads and mahogany keel. Even set my cased shotgun in its rack. I am taking some pieces over to a friend who has a band saw to cut the curve of the keel and some deck curves on the bulkheads.

This is all very exciting, it's fun how it becomes more of a boat step by step. My helper is also a computer whiz and he's going to help me with posting pics.

John
 
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