acetone. you can buy it by thee gallon, its essentially thee same stuff as nail polish remover, but its potent stuff, so keep it from your paintwork. but if the previous owner has used a petseal tank liner (petseal is fibre glass resin based) that isnt capable of coping with ethanol (petseal isnt) the ethanol will cause it to break up and flake off. its a ferkin pain, but all you caqn do is add acetone and leave it to do its job, keep swilling it round to get into the crevises and work its way through. i put a litre at a time in and after a week i tip it out into the garden burner and set it alight, theres no other way to dispose of it, and pop some more in. it will take you a couple of weeks to get rid of it all, but it will do it.