I have a tank that was sealed on a '62 Dart 7-8 years ago and it has been fine. It was boiled out first. I believe Red-Koate was used. That may be why the prior poster's customer had problems with his sealer: lack of adequate cleaning.
This is probably correct. Lack of proper surface prep has been the primarary cause of many a bad paint job blowing up. Contaminants left behind due to improper surface cleaning cause many paint issues.
Tank sealant isn't paint true,but the same procedures would apply.