Why cars rust

"Rustoleum"....it's US or RUST the underside of my 1990 GMC 4X PU looks pretty close to the van previously pictured........ During the muscle car ERA, automobiles undersides were essentially painted with the cheapest lowest grade Lacquer just to make it either black or similar to body color. For lack of a better comparison a grade of paint similar to what comes in a rattle can. One of my very first jobs, was to wash all the "front line" new cars every day, got out of school at 3 and washed till 6:30. sometimes they would switch out the wheels and we got to see the underside of many brand new cars. we also got to watch them being unloaded from the huge car carriers. pretty much every single car we saw back then had some kind of rustalready going on as they were being backed off the trailers. for cars that had been washed several times it was far worse, and these cars had not seen 60 seconds of road time!! Foreign cars were the absolute worst for this.....except for VW's and no body knew why? Later came to discover that VW was the only manufacturer that used the same grade of paint on the underside as they did on the upside, in fact it was the same paint because they were essentially painted stem to stern top to bottom all at the same time...Later this led to the domestics actually DIPPING their cars in primer.....point being as automobiles progressed thru their initial years then thru the war years they underwent many changes. The steel on some of the first vehicles was THICK, I mean REALLY thick and that is why they didnt worry too much about coating for protection paint was initially for appearance! As war broke out metals became thinner, still no real protection was necessary because they were still pretty thick up into the 50's, then "style" hit the scene and all the outrageous fins & curves could not really be made without thinning out the metal quite a bit to make the parts and keep the stamping dies all the way through a build year. the thinner the unprotected metal....the more the likely hood of rust. RUST is what metal is NATURALLY supposed to do when it is left unprotected....cycle of life stuff. in certain regions and under certain climates rust is far less prevalent but not omitted. in other regions and climates it is more prevalent.....protect the metal reduce the introduction and continuance of rust. you can never eliminate it!
Things you can do to reduce it, especially in the rust belt and in heavy salt water areas.
for classic cars
1. inner panel spraying with paraffin ( or oil )
2. undercoat with a tar , wax or oil based product ( needs to be redone every so often)
3. undercoat with a catalyzed paint
4. cat paint or powder coat the frame
5. DO not use water to wash your car!! use detail mist

for daily vehicles
1. ziebart or some similar kind of rustproofing
2. undercoating with tar, rubberized product, wax/paraffin
3. visit a car wash weekly that has a bottom sprayer

ALL you can REALLY do is delay the inevitable and fix it when it happens

my two centavos