Hi, my fishing dinghy has painted plywood seat bases some of which need replacing. The boat will be used in salt water and normally I wouldn't consider anything but a good quality marine plywood. However the original plywood is 21 mm and I just cannot find that in marine ply, 18 mm and then 25 mm yes but because of the way its fitted I need to get it as close to original as possible. I can find 21 mm Birch Ply, would this be Ok if it was painted in a 2 pack epoxy paint.