I have no reservations about powder coating an old gas tank. Take it to a radiator shop and have it boiled out. Silver solder any pin holes. The radiator shop can do this for you. Then have it sand blasted and coated. The coater can usually do the sand blasting. Finally, get an Eastwood gas tank sealing kit. This is a coating that you put into the tank and slosh around real good. It fills small holes and puts a rust resistant coating on the interior.