This is bathroom related so it may belong in the bilge. But the technology is boat related, so I'll put it here.
Three years ago I built a soaking tub out of 2x4s, slate, beadboard, epoxy and plywood.
I did a fair bit of research and there was a strong feeling that this was a stupid thing to do. It was especially risky because the tub is below all the plumbing in the house. So my tub has a bilge pump. The epoxy is West System Mirror Coat and the ply is 3/4 birch from Home Despot. The only screws are used create slope near the drain and they are bunged. I have recoated the tub with epoxy once after just a light buffing (the dog who is afraid of fireworks jumped into the tub before her pedicure.) There was some water incursion along the top edge below the slate because I used common bath caulk. I replaced it with roofing sealant and the problem went away. I clean the tub with regular house hold cleansers and after three years the tub is as good as new. And a 6'2" guy can completely submerge himself fully stretched out.
I thought those of you who like to push the edge with epoxy and cheap plywood would like to know.
Enjoy the glorious summer.