My chassis was painted with Rustoleum which I sprayed on the entire underside body and chassis with the compressor, I gave it a coat a week for 3 months to build it up with light coats, then spent two days with brake cleaner and an airline blow gun getting the Sahara dust and small pebbles out of the inside of the chassis, then bought 5 litres of Tetraseal and 10 bottle cartridges of Tetraseal Shultz, added petrol to the sealant to make it be able to seep into every nook and gap and spent another two days blasting that dirty oily dirty muck in to the chassis until it oozed from every drain hole and you could see it atomising out the drain holes. I usually like a painted finish on the outside of the chassis but this crap went everywhere at 150 psi so I have now covered the exterior chassis too and at first it looked awful, really glossy and thick, but after three days now its quite smooth, looks like a semi gloss and is paintable, oh and I removed the door hinge bolts and blasted in the bulkhead and B pillars, this time remembering to cover the bolt holes on the inside otherwise you'll cover the interior in black ****.
Rustoleum I've found needs about a 10%-20% mix of thinners to be sparyable, I think I paid £50 for 2.5 litres but if you spray with it, it goes a long way, seems very good at being chip resistant and clings to bare metal really well.

Good stuff, been around since 1921...... If it weren't good, it wouldn't be here in the 21st century

I've just put a new rear cross member on mine and decided to give the chassis some love. I've stripped it all back using a wire brush on a grinder, primed and painted with POR-15 then over coated with a couple of layers of underseal. The inside of the chassis i sprayed with 5 litres of waxoil thinned with some petrol and blasted in under pressure with a schutz gun whilst it's good and hot to make it runny. Forced air mask a must! I've done a couple like this in the last few years and they are holding up very well. I really rate the POR-15 and have been using it on bike builds for a while.