If you could get the pipe in a big bath (with adequate ventilation) you could reverse electrolysis it.. check this link...
http://www.frets.com/FRETSPages/Mach...stremoval.html
[Edit]Or... there's some stuff I've used called KuRust, but that's only for small areas really...
I've spray painted a rusty compressor with Hammerite but you need to get rid of all the loose rust (not completely clean it of rust)- it undercoats and paints all in one.[/Edit]
Hope this helps!