There are several points on your car that are more susceptible to rust. For example, your car’s engine, frame, chassis, exhaust system and trunk compartment can develop rust. Optional equipment, like trailer hitches, can also fall prey to corrosion. Rust can even attack painted surfaces on your car.
Rust is metals version of cancer.
Once it gets hold it takes over eating into more and more of the metal. Rust needs Oxygen and moisture to perform its stuff.When metal is protect by a layer of paint the oxygen cannot penetrate. However a stone chip is all it takes to start off the process.Initially rust will bubble up the surface of the metal gradually eating deeper and deeper. The earlier it is caught the better it will be.
A rust spot will need to be thoroughly treated.
Sand down the rust spot until you get back to bare metal. It is a good precaution to go a little wider than the damaged area just in case it has started to spread further. You may have to cut out the area and weld in a new plate. It is critical that you sand the surface flat as any imperfections will show up when it is painted.
Cover the whole area with a rust inhibitor which will act as an undercoat/key for the next stages. There are many treatments on the market but most do a very good job. They should not be viewed as a rust killer and remover though - they merely prevent rust from happening in future.
Fill the depression caused by sanding with filler, smooth and sand off. (If the depression is only small you may well get away with a thicker coat of paint or some high build primer.
Cover the area with paint and then finish with some cutting paste and a clear coat.
When the rust is a little more serious, for example on wings and the bottoms of doors your only real option is to cut away the damage metal and weld in a new panel. Doors can usually be replaced cheaply from the breakers yard and repainted which is usually easier than a repair.
Front wings are also easy to replace and paint.Anything on the rear wing and structural areas with rust will need a new panel welding in to properly sort it.Be sure to cut the rust thoroughly away and then cover the area with a rust inhibitor to prevent further damage from forming whilst you are working on the area.
Rust prevention is far easier to do than rust repair. The most obvious way to guard against surface rust is to apply a wax coating every 6 months. In addition to this there are a number of corrosion inhibitors on the market which are typically sprayed under the car and behind panels. TorqueCars would recommend this is applied in a number of thin coats. Special care should be taken to prevent the drain holes in the bottom of the panels from getting clogged up.
It would be a good idea to place a piece of blunt wire through each drain hole to ensure that it is not blocked. The last thing you want to do is to allow water to collect inside the panel or door.
Many leaks can be a sign of rust, usually they are just caused by broken rubber seals but the area should be fully investigated for rust. One frustrating area to get rust in is around the windscreen. We have known cars that have gone in for a windscreen replacement to require extensive work to the frames due to rust caused by ingression of water.