I used POR 15 on my spare, spare holder, cross members behind spare, front of the axle, drive shaft, etc. Pretty much anything that had any light rust. The rest of the undercarriage has been sprayed with Rustoleum yearly.

Here's a pic of the spare, came out well.

A quart of POR 15 goes a long way. I just use a coffee scoop and take out what I need. I leave the can of POR 15 in the refrigerator.

 

That stuffs great ! The rim looks new.

 

I love this stuff. Even though I was wearing gloves, I managed to get it all over my arms. I've scrubbed with a brillo pad twice, it's not coming off haha. Looks like in be wearing long sleeves and sweating for a few days.

I also really like the fact that it's self leveling. When it dries, you see absolutely no brush marks.