I know mercedes and a few other european cars used to use a strip of undercoating along the lower exterrior body metal and doors for a number of years to deflect rocks and in turn prevent paint from beinc chipped, which allows the metal to be easily accessed by salt and water leading to rust ( and most folks just thought the black stripe looked cool
it''s really a good thing to consider if repainting, but take it down to good metal and PRIME well
Thhen of course, there are the italian cars , that rust in a sealed room, just looking at them, Some vehicles were painted in poor moisture control conditions and it doesnt matter what you do , you will get rust. The jokers in Texas that make those outrageously expensive Western Hauler beds like I have should be shot for the crap metal prep and air filtreation they have in their paint shop. I've been working on my bed and finally came to the conclusion that I'm going to have to just sandblast the whole stinking thing down to bare metal because they shot in high humidity and didnt use metal prep before priming, the thing is one giant speckled rust monster waiting to come out , from under teh paint. ( and none of this has anything to do with salt, they dont use it around here)
