Virtually rust free my foot, if it hadn't been so long since I bought it from the guy on eBay I'd be wanting my money back.
While it seemed to clear up nicely and was rinsing clean, later a big chunk appeared and when I went to start stripping the paint I virtually punched through near the rear at the very top of the tank leaving about a 1/4" hole. Oddly enough when I probe around through the filler neck it all sounds and feels solid. IF this were the only hole it could be repaired but I don't think I'd trust it any further than I could throw it.
Anyhow, if someone needs the tank off a Custom as a "place holder" when they're trying to think up seat mods it's yours for the price of shipping.