Now for my method! First, find an old hair dryer, the type that had a hose on it. Discard the plastic head cover. Now, fill the tank with water, and empty. Do this three times. Now, insert the hair dryer hose into the filler hole. Turn on the dryer. The hose is usually smaller then the filler hole, so air can escape. let it completely dry the inside of the tank. Weld immediately. If allowed to sit for a while, the adsorbed fuel molecules on the inside surfaces may slowly evaporate. This can form a combustible mixture.
Steve R.