Originally Posted by Larry M
Prep has nothing to do with it. It's the nature of 2 part polyurethane paints. They are not made for constant immersion in water. Boat hulls painted with a 2 part with water lines that are too low can have issues. Decks painted with a 2 part that have carpet, if the carpet doesn't dry out will blister.
An epoxy, gelcoat or a designed paint would be my first choice.
What can I say. I seen/had bilges painted with 2 part polys and not seen any lifting.
But if its a concern, just paint down to say 6" of where any standing water might be and then paint from there down with something else.
Preferably, IMO, with almost anything but gelcoat. :-)