[chiphead]

What you need is a rust converter you can leave on and then top coat. A mild phosphoric acid like Picklex-20 or similar would work. It converts iron oxide to iron phosphate. Other converters like Naval Jelly contain phosphoric acid, but include other paint stripping chemicals like sulfuric acid that you do NOT want to leave in place.

I would remove loose rust and scale then treat with phosphoric acid. Sop up the excess and let dry. Then top coat with a primer product that is compatible with acid etch adhesion promoters. That usually means its compatible with phosphoric acid.