If you don’t want to go to the radiator shop, Eastwood products make a tank sealer.

http://www.eastwood.com/ew-gas-tank-sea … 2-oz.html#

I had used it years ago on my Tiger and have had no problems. I did have the tanks
tank dunked before hand so I knew they were clean. Don’t think I’d trust my radiator
shop with my gas tanks, had hard enough time to get them to fix the radiator.