Why don't you contact the people who make POR-15?
POR-15 also makes a stripper that works on other tank sealers, maybe it will strip their own product?
Their web page says this: "For those unfortunate customers who have had bad results from other inferior fuel tank sealers, we have developed these all new "HEAVY DUTY UTILITY & CYCLE TANK REPAIR KITS."
They feature everything found in our regular UTILITY & CYCLE TANK REPAIR KIT, PLUS a quart bottle of our famous Tank Strip to first remove that inferior coating that failed you."
I actually did that and they said that their POR-Strip was designed to get POR-15 off. However, after more than a few attempts, all of the coating has yet to come off (maybe only 30% at most). What bothers me is what I can't see (i.e. all the nooks and crannies in the tank)...and I wouldn't want anything breaking loose down the road and swimming in my tank or even recoating over a bad surface. So, I'm hoping an industrial strength chemical dip would do the trick. Plus that POR-Strip is nasty stuff and I don't want to personally handle anymore caustic chemicals.
I'm taking it to a shop tomorrow actually to have it done. I'll let people know how it goes. On another note...Should I recoat it, or just keep the tank topped off with high quality gas? After the last coating attempt, I'm a bit jaded and don't want to go through this mess again. So if there are any alternatives, I'd like to know.