http://home.metroweb.co.za/~pdoug/tips_and_recipes.htm
Take a ½ kilogram of paraffin wax or candle making wax and grind it up with a cheese grater. Soak it in 2 liters of mineral spirits/ Paraffin/ Kerosene/ lamp oil/ Diesel until all of the wax is dissolved. It might take a couple of weeks. Stirring will cause most of the wax to dissolve, but soaking should take care of the rest, heating the mixture in hot water (no open flames) will add in the quick dissolving of the wax. Generally try to dissolve as much wax as the mineral spirits/ kerosene/ paraffin/lamp oil/ Diesel will hold. Pour one liter of mineral oil/ non-detergent motor oil into the dissolved mixture. If the mixture is too thick for spraying you can thin it further with more mineral spirits until it is of a spray-able consistency.
Application:
Buy any cheap engine spray gun at your local auto or tool store that carries air tools. You can also use a garden insecticide sprayer, although it will really make you tired. Rent or borrow an air compressor. If possible elevate your car so that you can get at the underside. (Remember to chock both front and rear wheels as you are going to work underneath the car.) First wash your truck's chassis, both inside and out, to get rid of all the mud, oil etc. Spray the solution onto the underside of the chassis and into every little hole, crack and crevice. Make sure you push the tip into the chassis to spray the inside of the chassis. Let the excess drip off. You will find that the solution will blend into all cracks and chips and if the chassis is scratched it will flow, covering the scratch. If there is existing rust the solution will be soaked up by the rust and thereby creating an oxygen free coating stopping the further rusting/corroding of the surface.
Repeat the above once a year if you live in an area where the roads are sprayed with salt. Once every two to three years if staying inland where it is dry and salt free.
The above information was received from the LRO list where various people contributed their own recipe. Most of the recipes was essentially the same varying only on which oil to use and discussing the pro's and con's of Mineral spirits/paraffin/ Kerosene/ lamp oil vs. Diesel etc.