I will take a shot at this, but be forewarned that I am not a pro. I frequent a site called autobodystore.com in the forum section where a lot of pros do reply, so you may want to venture there.
First off, OSPHO is a brand name for a phosphoric acid product. Phosphoric acid converts rust to, I believe, iron phosphate. When you use phosphoric acid, you have to ensure that your primer doesn't also contain acid (i.e. etch primer and some hybrid epoxy primers)otherwise you risk delamination occurring. Normally, you use a squirt bottle, and mist on phosphoric acid, work it in with a scotch brite, then wipe it off with a paper towel. You should let it dry, then rescuff the metal with either a scotch brite or sandpaper before you prime. Phospo by itself only keeps rust from reforming if it remains dry. Therefore, you should plan to prime/paint any areas treated with it otherwise moisture will cause them to start rusting again.
On the primer, I wouldn't use etch for the reason above. I am using OSPHO, then epoxy primer. Epoxy is made to apply direct to metal and also to be used as a sealer before paint. The one I am using is Kirker Enduro (see smartshopper.com). It is very reasonably priced and after a few days is easily sanded without clogging sandpaper.
On the paint, I wouldn't recommend Duplicolor. I believe it is acrylic enamel which doesn't hold up very well. I would upgrade the paint to urethane in either a single stage or a base coat with a clear coat. The urethanes are a far superior finish.
I hope the above is helpful.
Regards,
Steve Lorio
New Orleans
