We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "The Medway Handyman"
saying something like:

Loctite do a product that apparently converts rust to something black, never
used it, heard its good though.
http://www.loctiteproducts.com/produ...id=47&plid=168

Probably phosphoric acid.
Dirt cheap to buy the raw stuff.