Working on some real old equipment for a career has been more than a pain in the patute. We found that a product named POR 15 works great. Particularly in fuel tanks that rusted to the point of scaleing inside. You know…. where little flakes of rust keep you busy cleaning/changing the filters. It bonds to the metal. Lasts longer than paint. We cleaned the tanks with muratic acid and then poured POR15 inside. No more rust. Used it also on exposed blades and frames that were either not cost expediant or just too much time/cost to get to for paint. 
Here's a link….. http://www.por15.com/POR-15/pr…..tinfo/1GB/
Jack
People are hilarious...
I'm supposed to choose and be in one of the 2 parties. And NEITHER is worth a damn!