All that Ospho is gonna do on the pitted rust is stop the chemical reaction to help stop it from continuing to rust. The pits will still be there.
I know for a fact this stuff is not flammable, because I used it on a section of my floor and then verified it wasn't flammable before welding in some bucket seat plates. Just double check with the mfg of your primer to make sure it isn't going to have an adhesion problem.