I've done a fair number of truck guns for people with oxpho, weren't worth the cost of getting blued. Constantly amazed at how well they came out. Got to try all the tricks in the books (Kinks) articles, videos, etc. Some worked, some didn't. IMHO whether it works or not depends on the individual's exact technique - you gotta play around with it a bit. (A benefit of doing truck guns, free practice pieces that can't help but come out better and make the owners smile. Win-win.)
I sand down until it feels right which is at least knock the rust off and sort of scuff sand to knock off any kind of barrier on the surface. I don't bother to sand off all old bluing. Apply Oxpho with 0000 steel wool, degreased because it makes me feel better though it doesn't seem to matter. Give it another light coat with a patch and let sit overnight. Then rub with dry 0000 to take off the excess "bluing" and blend. If streaks, light spots, etc. apply another coat of oxpho and repeat.
Cold blue is not a conversion coating like rust blue or caustic blue that "convert" the iron in the steel to iron oxide. They are selenium dioxide based and deposit a selenium compound on the steel which looks pretty.
