Most tank sealers are pretty tough to remove ... I do know that MEK ( Methel Ethyl Keytone ) can be used to cut ( thin
) sealer ( some types
) that have thickened in the bottle. MEK is highly flammable !! http://dhss.delaware.gov/dph/files/methetkefaq.pdf
I would presume that you will have to repaint the tank also because ( usually
) the cleaning of the inside of the tank and resealing is real messy and very hard on the finish. I'm afraid that's all the help that I have. Hopefully someone else will chime in.