I remember Brian telling me (maybe 10 years ago) that he routinely sealed tanks, but he didn't say what he used. He did disparage commercially available sealants. I have a hazy recollection he mentioned aircraft sealants, which would be odd because Avgas does not contain ethanol or methanol. My racer tank lasted 10 years on Avgas sealed with Petseal, which dissolved the first weekend (last year) that pump fuel and an octane booster was used. I have bought Restom sealant, which, even if the owner wasn't Dany Vincent, got to be good stuff, is French, and French fuel contains more ethanol than ours currently does. (Caveat: I haven't tried it yet. It works best applied above 15 deg C, and Aberdeen has yet to see such elevated temperatures...)
My steel tank was sealed five years ago with POR15. Still OK.
Brian Dees from Chatham sealed my tank many years ago, with what i don't know but up to .now i don't seem to be having any problems