the above picture is how the underside of my car looked when I got it. Scary huh? I really lucked out becuase it was all surface corrision, the floors and trunk and everything were still very solid. So I set out to get the flaky rust off and seal the underside in a nice POR-15 rust proof sealer.
In some ways doing the undercarriage is easy becuase your not going to damage anything mechanical and even if the painting is ugly, not too many people are going look at it, durability is much more important. But then again, some ways it is a pain, especially if you are poor like me and have to do it in a gravel driveway on your back! Your going to get really dirty and wet.
The first thing to do is rip off everthing from under your car (follow proper safety procedures for being under a car). If the underside of your car is as rust as mine, you won't be reusing much. Unbolt all the bolts, and sve anything you can. Then BEFORE you start scraping off rust, wash with a good degreaser. Do not use car wash detergent, use degreaser, like what is used before paint at a body shop. rinse well. If you don't degrease before wire brushing you will grind the grease deep into the metal, and your paint won't stick.
Then put a coarse wire brush on your electric drill, lay on your back WITH GOGGLES and start going at it, get as much into the nooks and crannies as you can, get all the flakes off. Then use a rust stopping acid (POR-15) and spray all over, let it soak follow directions. (the next picture shows my undercarriage soaking.)
Then flush all surfaces with lots of water, once the water starts to dry, you will see the rusted spots start to turn white as the rust stopping chemical's work is exposed. (see next photo)
After the rust stopper has all been rinsed off and your car is very dry, (any moisture will ruin the sealing job) replace all the bolts into the threaded holes to protect the threads from the sealer. Now, grab a brush and some clothes your willing to throw away, lay on your back, and slop on POR-15 sealer. No matter what you do you will get this on your skin, it will get under your cuffs and under your collar, and it doesn't come off until your skin naturally sheds, so don't do this the night before your wedding! I highly recommend a face shield and hat.
But if you do it well, you should have a sturdy undercarriage that is well protected from the elements. Good luck!