I sent an old tank up to Plasticweld (Bob Brown)
www.empiregp.com a few yrs ago to have the inside cleaned out and coated to prevent the rust from spreading. This tank was from a Yamaha Virago that sat for most of its life. was 20yrs old and had 1500 miles on it. The inside was nasty but no holes formed in it yet.
Got it back a week later from Bob and the inside coating that was applied was smooth as glass. Don't know what he used to clean it and seal it but it was good stuff.