I am a believer in Ospho, (that's the brand name of it). It's phosphoric acid. Once I blasted my frame, I immediately brushed on a coat of it. Turns black and acts as a primer coat. If you have a rusty area, doesn't matter. That's what it's made for.
Only drawback I have with it, once it dries, (overnight),the finish is not smooth, and it leaves a slight chalky residue. I always hand sand it with 150 or 180 grit, lightly, and then shoot a coat of primer. Smooths right out then.
I've sold it and used it for at least the last 25 years, and have always been a believer.
Mike
