Rustproofing  Rustproofing is the prevention or delay of rusting of iron and steel objects, or the permanent protection against corrosion. Typically, the protection is achieved by a process of surface finishing or treatment. Depending on mechanical wear or environmental conditions, the degradation may not be stopped completely, unless the process is periodically repeated. The term is particularly used in the automobile industry. In the factory, car bodies are protected with special chemical formulations, typically phosphate conversion coatings. Some firms galvanize part or all of their car bodies before the primer coat of paint is applied. If a car is body-on-frame, then the frame (chassis) must also be rustproofed. Paint is the final part of the rustproofing barrier between the body shell (apart from on the underside) and the atmosphere. On the underside, an underseal rubberized or PVC-based coating is sprayed on. These products will be breached eventually and can lead to unseen corrosion that spreads underneath the underseal. Old 1960s and 1970s rubberized underseal can become brittle on older cars and is particularly liable to this. Aftermarket kits are available to apply rustproofing compounds both to external surfaces and inside enclosed sections, for example sills/rocker panels (see monocoque), through either existing or specially drilled holes. The compounds are usually wax-based and can be applied by aerosol can, brush, low pressure pump up spray, or compressor fed spray gun. An alternative for sills/rocker panels is to block drain holes and simply fill them up with wax and then drain most of it out (the excess can be stored and reused), leaving a complete coating inside. Anti-rust wax like phosphoric acid based rust killers/neutralizers can also be painted on already rusted areas. Loose or thick rust must be removed before anti-rust wax like Waxoyl or a similar product is used. Structural rust (affecting structural components which must withstand considerable forces) should be cut back to good metal and new metal welded in, or the affected part should be completely replaced. Wax may not penetrate spot-welded seams or thick rust effectively. A thinner (less viscous) mineral-oil-based anti-rust product followed by anti-rust wax can be more effective. Application is easier in hot weather rather than cold because even when pre-heated, the products are viscous and don't flow and penetrate well on cold metal. Aftermarket "underseals" can also be applied.