There must be no traces of rust, old paint, and rustproofer on any body part you want to rustproof. Besides sanding and sand blasting, there are other methods such as having a professional dip the part in a caustic solution. For smaller areas and part, you can use a rust converter, which turns the rust into a black material, or a rust remover, which will annihilate any traces of rust remaining.

It is best to treat all bare metal steel or aluminum surfaces as soon as you can after they have been stripped to prevent any rust or corrosion from forming. Spray protectant in aerosol cans is great for this because they go on fast and there are no brushes to clean up afterwards. You can use an anti-rust primer first, if you prefer. A zinc spray works well and is a good coating for steel and aluminum. This is because zinc actively discourages rust from forming. Think galvanized steel.

Rubberized body sealers adhere well and are great for sealing seams and joints.