I have reviewed previous discussions concerning rust but none dealt with exhaust manifolds.
My Cummins manifolds and exhaust have considerable rust on them. Can the phosphorus compounds be used on them? I don't know if I would even try and paint them. I am not taking them off (If it ain't broke don't fix it).
Any advice will be greatly appreciated.

Have you perused Hemmings Motor News?

Lots of ads for exhaust paint there. www.Eastwood.com is one.

My own preference is to sandblast first, as some rust cleaners leave a chemical or acid residue in the pores that messes up paint later when it gets hot. If I were to use a cleaner, I would get it real hot for a while before painting.