Coatings parts for rust prevention

"Take a pound or so of paraffin wax and grind it up with a cheese grater. Soak it in a half gallon of mineral spirits until all of the wax is dissolved. This might require allowing it to sit in a closed container for a couple of weeks. Stirring will cause most of the wax to dissolve, but soaking should take care of the rest. Generally try to dissolve as much wax as the mineral spirits will hold.
After that, dump in a couple of pints of mineral oil (less of a smell) or non-detergent motor oil. If the mixture is thick, thin it further with more mineral spirits until it is of a sprayable consistency.
Buy one of those cheap engine sprayers at the local auto or tool store that carrys air tools. It is a metal wand with an air fitting, a spray button and a rubber tube to dip in whatever you are spraying. Rent or borrow an air compressor.
Safely elevate your car so that you can get at the underside. Spray the solution onto, and into every crevice, crack, hole, and surface you can get to. Then let the excess drip off. The beauty of it is that the stuff will soak into any rust spots. The rust seems to hold it like a sponge. It will also bleed into the smallest cracks and folds of the body and protect there, too."

Fogging spray might even work ok.

Soundguy