Phenolic resins are not truly waterproof. I would use an epoxy resin such as WEST epoxy (not related to West Marine, but you can buy it there) or System Three. It takes about three coats, first one cut with thinner around 10% (good quality lacquer thinner is OK). I use 4 oz glass cloth and epoxy if the surface needs wear resistance. Problems with water usually occur through the edges, and I like to use fiberglas tape on the edges as well. If you are covering with something like Nautolex, forget the glass. Epoxy is not sunproof, so must be painted.
That said, Karl would be the expert on using Petit products, I wonder if he will post his writeup?
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida