I used EPDM for my 800 gallon hot water storage tank, now approaching 10 years of problem free service, but I have seen coated tanks at builditsolar.com. I would not use epoxy paint over plywood, epoxy- fiberglass, perhaps. There's and article there somewhere on coated/parged plywood; one using a silicone product. Long term durability is the concern, especially with higher temperatures.
https://www.builditsolar.com/E...
Economical lined barrel type tank using a special plastic liner:
https://www.builditsolar.com/E...
The photo is my own 800 gallon mostly below grade tank. The tank being below grade saves space and supporting structure as earth lined with cement boards, then foam boards (just tamped sand and foam on the bottom) are used. The strong insulated lid allows me to use the lid space for shop storage.
