I just recently did my undercarriage. I used
Corrosion Prevention | Waxoyl USA products in combo with some
RustOleum.com.
I wire brushed what was necessary. Then I sprayed clear Waxoyl on the painted body colored sections of the underside. I also sprayed clear over all mechanicals, hoses, pipes, fittings, crankcases, bumpers, suspension, diff, brake backing plates, etc. Everything except the exhaust. You could use black but it was an aesthetic preference for me.
I then spray painted black rustoleum paint on all the frame and exposed nuts/bolts etc. I did this just for additional protection. Cheap and easy. Finally I sprayed black Waxoyl on all the frame and associated hardware.
What I like about Waxoyl is that it sprays very evenly, has a perfect matte sheen that looks like paint almost and never flakes or falls. off. Doesn't collect dirt either. I have used this on many classic car restorations and love it.
To make the job much more manageable and easy and neat, I used a portable spray gun designed for such jobs. It has a bunch of attachments such as long wands, curved wands, a wand on a hose, and several wand ends that give different spray patterns. Makes it super easy to get everything covered. I even shot up inside all the frame openings. That's important to do. I got the gun years ago at Eastwoods. I use it for all sorts of jobs. Get one!
Well, I did use gloves then after a rest I forgot to put new ones on. Oops. But this was as dirty as I got.
Black and Clear waxoyl. Virtually everything you see is covered and protected now.