i have torn apart 3 different trucks that were z barted and the spray pattern of the coating they used was only about a 4 inch circle behind every hole that was drilled, the vast majority of the sheet metal inside never even saw the coating. That of course was due to the poor workmanship of the person spraying. Regardless, all the trucks were super rusty. I do live close to the gulf coast however, and theres nothing you can really do to stop salt air. Our local zbart has changed hands so many times because it is so ineffective. It is notorious down here for being a scam. Really, unless they are getting every little crack and crevices, the places where the rust really is a problem, how is it going to work? Also, if you are spraying a used vehicle, the coating is going to be sprayed over dirt, sand, moister, or whatever is caught up in the seems. It will then just hold those rusting agents in. My opinion might be biased because of my circumstances, but It just seems to me that all you are doing is paying somebody to drill a hundred little speed holes in your door jams...