They use this wacky oil based paste the comes out of a nozzle through a pressurized hose and spray it all over the inside of the rigs body. They do this by sticking the nozzle through holes drilled into the inner door frames and by removing the rear brake lamp housing and in all the seams or lines of the body.  They even spray the under carriage and wheel wells.
This stuff then sort of congeals and then becomes rather water proof and more importantly SALT proof and don't have to worry about salt for another year.  Some Sidekicks from 95, 96, 97, and even 98 are WAY more rusted then my 91 is.