I've used POR15 tank sealer on several tanks at our museum, the only one I had problems with was when someone told me they had cleaned the tank per the instructions and didn't. And that stuff is the devil to get out. MEK didn't even want to cut it.
Prices on tank cleaning vary a bunch. Around here there is now only one outfit that will boil a tank, and he has a several week waiting list. Charges $100 just to clean the tank.
