Most of the times the sellers of tanksealers also sell a product to remove defective sealants chemically. POR15 might do so to.
See: https://www.tankcure.nl/Engels/voorbehandeluk.html on the bottom of the page for an example of such a remover.
I have used “tankcure” sealant in many tanks so far, including my Flyer tank as well as my two speeder biscuit tank. I never had any experience with flaking or bubbles and am very happy with the results so far.
As regarding to Grahams tip: I think I remember Colin advertising somewhere quite a while ago that he had only one tank left….