Update on clearcoat vs spar varnish for UV protection

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cedarphile
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Joined: Sun Jan 22, 2017 11:41 am

Update on clearcoat vs spar varnish for UV protection

Post by cedarphile » Fri Jan 27, 2017 7:06 am

Clear coat UV protection: I'm eight years into a non-scientific test on the UV protection of 2-part polyurethane auto clear coat vs spar varnish for epoxy/fibrerglass substrate, and the results are encouraging.

I used 1 x 1 cherry strips to glue up a tailgate panel for my utility trailer. I coated the outside of the panel with fg cloth and epoxy, sanding it smooth to 120 grit like one would do on a cedarstrip canoe. My body shop sprayed on 3 coats of 2-part polyurethane clear coat in just over an hour, letting each coat flash-dry for about 20 minutes. The inside of the panel was painted with 2 coats enamel paint so seal out moisture.

For eight years, I intentionally parked the trailer with the tailgate facing south to get direct sunlight. It got splattered with sand and gravel occasionally when I was sloppy with the shovel at the gravel pit. I backed into a couple things, and the panel of course got rained and snowed on for eight years.

The only difference I can see in eight years of abuse and direct sunlight is that the gloss finish has become a satin finish. There's no deterioration of the epoxy, the fg cloth is not turning black as happens with spar varnish after 3-4 years of not recoating, and the clear coat finish seems pretty near bulletproof. I now use clear coat on all my strippers as I'm sold on it.

The often-mentioned difficulty of repairing scratches in super-hard clear coat has now been solved with the development of brushable clear coat with even more UV inhibitors than sprayed auto clear coat. I get mine from Progressive Epoxy Polymers, Inc. The brushable clear coat is called ACRYLIC POLY UV PLUS. It's brushable, but only for about 30 seconds after lifting the brush out of the mixed liquid. Rolling with a foam roller is the preferred method, giving about a minute of working time. With either method, the finish is high gloss. It can be made into a satin finish by rubbing with very fine steel wool within an hour after application. After that, it's too hard.

One caveat: if you're panning to spray clearcoat directly onto wood which has not been covered with fg cloth and epoxy, and sealed on the other side, be sure the wood has been kiln dried. I had an air-dried white oak transom plate on a fishing boat sprayed with clearcoat after just rolling on two coats of an epoxy to seal the wood from moisture infiltration. Left in the sun for a week, unsightly large bubbles popped up, the residual moisture in the air-dried wood rising to the hot surface. I had to sand and recoat the wood three times before the bubbles quit coming out.

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