I used to use Krown rustproofing when I owned vehicles vulnerable to rust (1979 Chev full size station wagon, 1991 Pathfinder). I had them done every two years. Basically I found this did not halt rust completely, but was 80-90% effective.
I found that if you drive on dusty roads much, as we did, the dust cakes into the Krown layer, which then separates from the metal as a separate layer, allowing moisture and then rust underneath. I had gas tanks and even gas lines rust out despite the treatments. The Pathfinder still suffered from the typical rust areas they have - just not as badly.
The stuff dripped off for days, so I'd park on the street instead of the driveway or in the garage. Making the problem a public one, not mine:-) But replacing rusty cars has environmental costs also.
Overspray on window guides was always a problem. It made the engine compartments such a mess that mechanics hated to work on them. Rubber components would swell, but did not seem to suffer early failure due to the treatments. The rustproofing did work as sort of an anti-noise treatment.
All in all, the treatments were vastly cheaper than the rust repairs that would have been needed otherwise. I'm just very glad that I now own cars that are almost invulnerable to rust. One of them is a 1990 Dodge Spirit, on which I just replaced the original muffler, and still has the original pipes.