I have put several thin coats of varnish on the hull of my Eastport pram and have sanded it with 440 wet sandpaper between each coat. What I am not sure of is how much varnish I am removing by sanding? I am concerned if I am removing too much there won't be enough varnish left behind even after five coats to provide adequate UV protection. I sand until most of the gloss of the varnish is gone with some shiny spots left, but I still feel like I am taking most of it off. Is there any way to tell how much varnish is on the hull?