Ospho before POR ?

Have to do some rust repair.

Should I use Ospho before POR or just go straight to POR ?

Use one or the other. They are both coatings and have been tested for what they are intended to do. 

Once the POR is on you want it to stay on. I'm not sure how Ospho would effect that. 

POR's "Metal Prep" or whatever it's called is the same chemical as Ospho. One or the other is necessary, but using both would be redundant. 

Ospho when applied to steel forms a black/gray oxide on the surface. Paint adheres well to this oxide layer. POR actually recommends applying to a scale free but lightly surface rusted surface for best adhesion. POR 15 requires an overcoat of paint to protect it from UV rays that will discolor it and eventually allow the POR to break down. POR is hard to remove from skin so wear nitrile gloves. Someone here will certainly chine in with a warning about getting POR on other body parts.

Yeah, don't get it on your hootus.

In reply to Crackers :

Thanks, I bought the little 3 part kit so it looks like I'm good to go. Just need a day with no rain and reasonable humidity.