I have used it several times on the underside of cars; the most recent is a 69 Barracuda which I just finished.

On a previous cars that were done a couple of years ago ir did it as follows:

I brushed the por-15 on the chassis; within 72 hours of por-15 application I sprayed primer; after the primer dried I sanded the primer; then sprayed my basecoat/clearcoat.

It held up very well.

On some 60's GM vehiles they were deliveried with only primer on the undersides of the body. Putting the por-15 on first to protect the body and then the primer for the proper look finish.