Tom,

I have used it on my "plastic" ski boat for the past year or so and found it to be the best thing I have ever used for UV protection. The boat spends all summer in the Eastern washington high desert environment and really used to take a beating from the sun. I put it on my kayak last week and have seen no visable affects to the varnish, but as Lee pointed out it makes the boat very slippery when wet.

Nick