If the rust isn't so bad it's loose, you can fix it with Caswell epoxy tank sealer. The rough surface provides a better surface for adhesion. If it is that rusty, just clean it and use the Caswell. Tape over the pinholes before putting the epoxy in the tank. I've used it and it works well.
http://www.caswellplating.com/restor...nk-sealer.html
I also have a couple of less than perfect tanks in yellow and blue.