I have had bad luck with Helmsman Spar Urethane. I have a redwood picnic table that I keep clear on, and Helmsman only typically gives me about 2 good years. However I have never tried Helmsman on top of a different product, nor have I ever applied more than 3 coats. In the test I was reading, they used 8 coats and sanded the sharp corners rounder.
Clearly, I need to read those comparison tests more, when I get more time to sit down. I wish they tested an automotive paint clear coat. That stuff will last many years on car paint, so why wouldn't it do the same on top of wood?
