I'm a little late to the party, and it may have been mentioned above, but an excellent alternative to the expensive epoxy paint is PPG's Coal Tar Epoxy.
I put this on my sink when I built it (3/4 inch regular plywood) and have had zilch problems with it. It's *real* heavy-duty stuff that is used where you want to coat something ONCE and never have to do it again; I think I read somewhere it's used on the inside of septic tanks...
I used it because one of the 'how to build a darkroom' books I was using recommended it. I have not been sorry; it's held up very well and I cannot complain about the cost ($25/gallon?), which easily allowed two coats of my 7 foot by 2.5 foot sink. The paint was not available 'off the shelf' here in Des Moines, (it's an industrial, not a consumer product) but a nearby dealer ordered it for me and had it delivered to my door.
It comes only in black, it's thick (imagine what you would get if you mixed coal tar and exoxy together and you will not be far off the mark), but it works well.
If you're looking for a tough paint and don't want to spend a fortune, you might give it a try.