I've used it on bare steel as an etchant, but I think its primary use is for treating rusty steel. I understand that it can be used prior to finish-grade painting, but I've only used it on heavy duty stuff like car frames & suspension parts. A few tips:

If there is any rust on the surface, the Ospho will tend to form small bubbles which harden when it dries, so you may need to sand again just before painting. I think that may be necessary anyway, but I don't recall exactly.

Be sure to check with the paint shop before you apply it, it may interfere with their process.

Wear old clothes or a disposable coverall when you use it, it's very thin and you'll get little droplets of it on your clothes which you won't notice until the fabric dissolves (it's pretty caustic...)