Date registered: Jan 2006
Vehicle: 73 280SE 4.5, 71 300SEL, 03 BMW Z4, 72 Fiat 850, 80 Triumph TR7, 85 Porsche 911
Location: DC Area
Mentioned: 1 Post(s)
Quoted: 95 Post(s)
Do it. And do it twice on Sundays. Once you get one rock chip in an unprotected underbody, rust will set in immediately, and you'll not know unless you get under the thing.
If someone else is going to do the job, they should be working on a dry car. Assuming there's already some protective coating there already, I'd assume they would remove all dust, sand and oils, rough up the surface, and reapply/apply the new stuff without getting the coating on pipes, wires and other undercarriage bits. If you know of or suspect rust already set in, it's worth your while to remove/repair the stuff before redoing the undercarriage.
This was a fairly common thing in the 70s and 80s (I remember tv commercials for Ziebarting the undercarriage), and is common practice on modern cars. The same process applies to truck beds - a provider tapes off the areas to be protected, thoroughly cleans the preps the bed, and sprays the stuff on.
This isn't necessarily a tough job either - I've done this to a W114 I used to have, and my current 108. It's easiest if you have your own compressor and spray guns, but buying cans of the stuff works as well. I've seen people apply it using a brush too. Gallon cans of truck bed coating are about $80 at my local parts store, and can be reduced with acetone (nail polish remover) for easier spraying. I see truck bed coating in spray cans at the local hardware store for about $5 per...