I've used Caswells on several tanks with excellent results. I bought a new FG fastback tank from Burton Bike Bits in 2008 and sealed it with Caswell sealer. No issues of any kind in the 10+ years it has been in service. BUT...this was a new tank that I sealed before it ever had fuel in it so it's not quite the same application. If it was me I'd certainly give it a try - worst case...if it doesn't last, all you have lost is the price of the Caswell. Here is what I would do:
Make sure the tank is really clean inside before coating. I'd use 2 cups of solvent like lacquer thinner in the tank, then shake it and pour out. Lacquer thinner MIGHT attack the paint, though, depending on what type of paint is on the tank. If I was concerned about that, Naptha is a good choice - pretty good solvent and generally won't bother paint. After the solvent is out and the interior dry (shoot some compressed air in if you want), dump 2-3 handfuls of short sheetrock screws into the tank and shake it thoroughly in all different directions for a few minutes then dump out. Then a couple of cups of soapy water to rinse out any debris followed by a thorough HOT rinse water (and more compressed air if available). Let dry and coat with the Caswell. Follow the Caswell directions!
