I have used Plywood (in fact mahogany underlayment)for outside in the weather applications with some success. I bonded two layers together with Gorilla Glue and then gave the whole several good coats of epoxy )West Systems). This material is impervious to almost any weather conditions. I should point out that Erie PA, which is where my boat and I live ranks No 1 in the nation in "Precipitation Days, average 188 days per year.