There is a product - phosphorus based I think - And available at home
Depot, that will convert rust into a chemical that protects from further
rusting. Can't remember what it is called. I used it many years ago on the
steel beam bumpers on a motorhome. It converted the rust, turning it black
and protected from further rusting. Painted it on, left it for a few
minutes, then washed it down with water. Finished as black, I never
bothered to paint it. I have some surface rust on my current van, and if I
can find this treatment, I plan to use it again. but my question is - would
it be appropriate as an undercoat for repainting my van. I certainly
want to stop rust, BUT I wonder about the effects on paint.

Has anyone else dealt with  this  stuff. It was very simple to use, but I
had my wonderments about its use before painting. Anyone? How did you deal
with surface rust before painting.