Just picked my tank up from the radiator shop. They told me they can't line it because there is a hardened sludge in the bottom of the tank that they can't remove. It appears to be something from the old gas that sat in the tank for 20yrs before I got the bike. Any ideas what the stuff is and what might dissolve or remove it short of trying to scrape it off with a long metal bar or something? They got most of the rust out, enough that the Caswell's epoxy I have coming should have a good rough but not flaking surface to adhere to everywhere but the very bottom. I don't want to risk the stuff crap on the bottom breaking loose and ruining the epoxy coat though. :?