Finished the restoration on my Velocette a few months ago and am currently working on 3 OSSA fiberglass tanks, so have explored most possibilities. If your steel tank has light rust and no leaks to worry about advice I got from Don Hutchinson (the Triumph tank guru) was derust and leave it alone. I used Metal Rescue which worked very well (tank must be cleaned first, soap screws, etc.). It's expensive at $25/gal. and takes 24 hr., but does work. Caswell's can be difficult to get a long lasting bond on steel unless the directions are meticulously followed, especially in regards to temp. and cleaning.
On old, funky, oil fouled fiberglass you have no choice, an epoxy product must be used. It's the only thing that will seal off the old constantly gassing off and degrading polyester/styrene resins. To get glass clean enough you must use a solvent, I use laquer thinner, but acetone may be necessary. When using Caswell's I always give it a full week to fully cure before exposing it to gas.
