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As long as car body panels are made from any type of ordinary steel, the metal will always try to revert to its more natural stable form, iron oxide. We normally just call this rust!

Moisture of any kind which can access the bare metal will soon form a rusty coating, which then penetrates deeper into the metal until eventually all of the original material is converted to iron oxide. (Most of this article will also apply to aluminium panels.)

Several methods are used to prevent rust getting a foothold, and also to slow down any existing corrosion.

1:/ Paint:

This has always been the primary protection from corrosion, the objective being to stop moisture/water coming into contact with the metal surface.
New or bare metal should be thoroughly clean before apply any primers - degreased and also treated with phosphoric acid `metal conditioners’ where needed. Note that if new panels are in use, any cataphoretic primer should not be removed - it is a factory applied anti-corrosion coating. More info in this section.

Good adhesion is the first step in protecting new or clean metalwork, which is why a good quality etch primer is essential. Following etch primer, a further protective layer of primer filler/surfacer will be required. Note that quality 2K products will nearly always be more resistant to moisture than, say, cheap cellulose primer filler. (Many cheaper products use chalk/slate filler powders which in themselves can tend to absorb moisture)
The same requirements apply to top coat materials - 2K paints will always be more weather resistant than older cellulose materials.

2:/ Stone Chip Coatings:

Many painted areas, or chassis and underbody, may benefit from a heavy, flexible coating which is able to absorb or deflect minor damage from gravel/stonechips etc.
These are available as solvent or water based, and should be applied after initial primer stages as indicated above. Most can be overpainted when dry, so can often be used on sills, exposed lower panels or other areas prone to stonechip damage.

3:/ Underseal/Body Shutz

A heavy bitumen based material, it is often used for underbody areas, and can often be found being used to camouflage existing rusty areas or repairs!
Not the best material to choose for slowing corrosion, as it can often crack as it dries out, and some early brands used mineral fibres as a filler which could actually act as a wick to lead moisture to the metal surface.

4:/ Wax Base Protection

Heavy wax products (e.g. Waxoyl) are ideal for rust prevention in areas which are not affected by wear or abrasion. The waxy, fluid nature of the products makes them ideal for protecting inside panels, door and chassis members. Because the material can soak in to absorbent surfaces, it is one of the few products which will actually slow down existing corrosion as it can exclude moisture from seams or crevices which are almost inaccessible to thicker materials.
A very similar material is often used to protect new vehicles which are in storage prior to sale 🙂

5;/ Electrolytic Corrosion

Electrical currents generated between dissimilar metals can accelerate corrosion. A good example is early Land Rovers, which suffered from heavy corrosion where aluminium skins were crimped over steel frames, i.e. doors etc.
This problem was partially alleviated when LR began to place a PVC film barrier between the crimped joins - perhaps something repairers could check for if replacing door skins or similar on older Landrovers or any vehicle using aluminium panels.