I used to rust proof cars at a shop I worked at, just did the new truck at in laws shop in Northbay, done yearly it does help big time.
I like the dripless wax/oil bases sprays, they are not runny, it's not about putting it on thick, but where you put it. Only way to do the inner of doors and trunks is either remove door panel and spray, or drill one hole and use a 360 tip on a long tube. Make sure you spray the hole and use the black caps cover the holes
Do not drill and spray in the rocker or a/b/c pillar, and try not to get it on the exhaust when doing the bottom