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The main problem I see with this is the mixing of mixing epoxy and polyester is they just don't like each other much. Polyester gelkotes, for instance, will not cure over epoxy. Epoxies over polyester generally fail after 2-3 years if moisture penetrates the polyester (I've seen lots of this kind of failure in the boat repair biz). Polester, BTW, is not as "water tight" as epoxy, so moisture does penetrate.
Again, everything is relative. If short term durability is your only concern, go for it. If you want to hand the boat down to your grandchildren, don't even think about it.
Unpigmented polyester is not very UV stable. That's one of the reasons you never see a polyester product without pigmented "gelkote". Unpigmented polyester resin usually have a rather unpleasant color. Painting solves both that problem and the UV problem.
Another problem with polyester resins is that they are naturally "air-inhibited" which means they do not cure in the presence of air. This makes the surface tacky after the "below the surface" part cures. In fiberglass part construction, this permits more laminates to be added without much surface prep, but makes sanding real tough. To get a surface cure, you have to add parafin wax to the resin (called a waxed resin) or cover your work with something like polyvinyl acetate (PVA). Either is a pain. Wax interfers with painting and is tough to remove. PVA has to be washed and scrubbed off. Compared to non-blushing epoxies, this makes for a fair bit more work.
Gelkote, could of course, be used instead of paint, but it normally gets it's gloss from a mold surface. When sprayed on like paint without a mold surface, it is dull and lumpy. It can be wet sanded and polished, but jeez, why bother. It's heavy too.
Keep in mind though you can use varying amounts of MEKP to control the polyester cure, most resin manufacturers limit this to 1% to 2% by volume. That means if one drop isn't doing it for you, you cannot got to six. More or less than the recommendations makes for goofy cures and physical characteristics that do not meet manufacturers spec. You can approximate the control polyesters give you by using epoxy resins and hardeners designed for different temperature. You can even blend some (ala MAS EPOXIES).
Craig
> I have been considering using polyester resin for making kayaks. My
> reasons being reduced cost and the (perceived) convince of being able
> control the setup times.
> So in reading in-between the lines, for a S&G kayak I should use epoxy
> resin for filleting the seams (adhesive strength) and sealing the wood
> (better moisture resistance), then apply glass and finishing coats of
> polyester resin for reduces cost and improved UV protection.
> Questions: - Does this sound like a reasonable approach? - Is there and
> compatibility (epoxy/polyester) concerns? - Is the UV protection of
> polyester good enough that painting would be optional?
> Hank
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Guillemot Kayak - High
Performance Wooden Sea Kayak Designs
