So here is what I found. My paint store stocked Coronado brand polyurethane Enamel. So that is what I used. The gallon cost $28.00 with no color added to it. I used no stain. The trailer sides were 1/4" cabinet grade Mahogany with Maple on the back side. (Not Marine plywood) The Woody Trim was made of Soft Maple. I used 2 coats of finish, sanding with 320Grit, between coats. I did not thin either coat. I applied the finish to the Mahogany with a small foam roller, and brushed the frame, coating both sided before attaching the Woody frame to the Mahogany sides.
The base is a milky gray color. It shows up a little on the Mahogany in bright sunlight. There are a few spots that have a slight ghosting. On the soft Maple or any light color wood this would not be a problem. The finish was soft, and took much longer to get hard, not like Minwax Helmsman or normal polyurethane clear finish.
This sample was made before I used it on the trailer. It has been left out to the weather since last April, almost a year. It has 2 coats on the mahogany side and no finish on the Maple side. It has laid on the ground, been rained on, sun baked and been under the snow. This is what it looks like today.
This is the finished product. Right now the trailer has about 4K miles on it. Last month I put on a third coat. If I had the time when I built it I would have put a third coat on it then. Over all I would say I had a good experience and the cost can't be beat.
