Ken, the owner of Whatcom Radiator in Bellingham, WA does an excellent job of reconditioning gas tanks. He does request if it is exceptionally dirty to take it to a car wash and give it a blast of hot water first to get the heavy stuff off first. Then it goes into his hot tank full of caustic solution for a long cook. Out of there, he inspects the tank carefully to make sure it is cleaned good enough. If not, it goes back in for a second bath. When the tank is cooking in the vat, varnish fumes are very strong as they flash out of it.
He then dries the tank thoroughly, using a hair dryer to move warm air through it. Once it is dry he then uses the tank liner solution to slosh the inside with alchohol resistant sealer. I believe most tanks get a couple coats. I have also brought in my gas filler pipe, wiring it to the tank so it goes along for the treatment.
I don't know of any shops in the Seattle area that do this, but I would imagine there must be some. Remember, an old timer told me many years ago a good radiator shop can make you or break you. Be sure to talk over the process with the shop to make sure they will do a good job and maybe look at tanks that have been cleaned and lined at the shop. This is a very good investment IMO.
