Rust proof your car for free

Adapted from the 2 hour New Zealand DVD Repair your car bodywork and spray paint at home”

The cause of automotive rust

Most perforation rust (perforation means the rust has eaten right through the metal) starts from the inside of an automotive panel, not the outside. What you see on the surface is usually just the tip of an iceberg.

Any observant motorist travelling overseas in a warm, dry climate soon notices the absence of perforation rust in the local vehicles. Obviously moisture is the major cause of rust.

Moisture from dew, rain, or condensation becomes trapped for long periods in dirt crevices, especially along the bottoms of doors and around the inner edges of guards. Even without dirt, lack of factory paint can result in perforation rust, especially in car roofs due to condensation. Parking your car outdoors every night without shelter in rain and dewy nights can speed up this process three fold.

So how how can you stop existing rust and prevent new rust long term?

Preventing rust

How often you have seen a rusty steel gearbox, or a steering knuckle? Or when was the last time you heard of a car engine sump rusting through? Most likely never. But these parts are all subject to moisture, salt etc, and are bare unpainted steel.

Why don’t they rust? The answer is oil and grease. These parts tend to pick up a thin layer of oil or grease. Oil and grease repel moisture. Rust is not a living thing like cancer, it requires moisture to continue. Deprive it of moisture and it stops forming, or never begins in the first place. This is the basis of the free rust-proofing secret.

Smear some oil or grease on a piece of metal and try washing it off with water. Virtually impossible without detergent. Whether the oil or grease is dirty or clean it still has this same property of repelling water and sticking to metal.

So all you need do to ensure that existing rust does not continue, or ever take hold in the first place, is to treat the inside crevices of automotive panels with oil or grease. Oil is easier to work with and has the added advantage of continuing to creep deep into crevices and cracks for months afterward.

Don't waste money buying new oil. The oil drained from the engine or transmission of your car will work fine. Most garages will fill a can for you if you don’t like to change your own oil.

How to do it

Using oil for rust proofing must of course be left until after any intended rust repair and painting is completed, or the adhesion of filler and paint will be seriously affected. 

(If you want to stop rust in the meantime, etch paint primer both sides will generally do the job, and also provide an ideal key for filler on the outside.)

A paintbrush is usually all that is required to apply oil to the insides of doors and other easily accessible panels. But some areas will require a bit of ingenuity. A spray-gun can be useful.

 However most oils will need to be thinned 10-15% with kerosene in order to spray. You can buy an accessory for the Wagner airless electric spray gun that consists of a long thin plastic 6mm (¼ inch) tube with a tiny spray nozzle in the tip. These are ideal for rust proofing hard to get at places like inside sills (the two bottom panels that run along below the doors) and roof edges. Just drilling a hole in a concealed area to gain access is all that is required

Whichever method you use, don't worry about overdoing it. Saturation is the key word. Surplus oil will find its way out, just be careful where you park the vehicle for a few hours. 

Oil may continue to seep out of cracks and drain holes for days afterwards but normally does not drip and can easily be wiped off with a rag.

Sills can be difficult to treat. Sometimes kick plates or carpet can be removed and holes drilled, or cut with a chisel, and a spray gun used (see above). If this fails, a professional rust-proofer can treat them for you.

In easily accessible areas such as boot floors, or where a dry finish is desirable, etch priming and painting both sides of a panel can be the best solution.

Exterior crevices

Deep exterior crevices which are sometimes found around windscreens etc, trap salts that can cause surface rust. This is especially noticeable in vehicles exposed to sea winds, or in areas subject to heavy air pollution. 

A simple way to prevent this type of rust is to fill these crevices with grease. The firm water resistant type designed for marine use is best. For a cleaner finish you can use a clear or black silicon. (Use masking tape along either side of the crevice when applying for a neat finish.) 

If rust has already started, a combination of both can be used. Make sure the crevice is dry, then forcing in oil or grease deeply to saturate the rust and deprive it of moisture. Then thoroughly clean and dry the surface area and apply a thin layer of silicon to seal it in. Again use masking tape to keep a clean edge.

Welding in new steel versus filling?

Most motorists believe that the only effective way to rid their vehicles of rust is to weld in new steel panels. This is effective if the new metal is butt welded, ie, welded edge to edge to existing panels without any overlapping. However this is seldom done as it is too time consuming and costly.

What usually happens is that the new panels being welded in are overlapped with the old and then the edge of the new panel welded on the outside only. This however sets up ideal conditions within the overlap for continued rusting, and also makes it difficult if not impossible to rust proof effectively. The chemical reactions of welding can also chase rust along even faster. Often within two years the rust is showing through again.

Whereas filling the rusted areas with a good quality water-resistant filler, applied over an etch primer or sound paint, not bare metal and rust proofing afterward can result in a permanent repair. 

However for areas that are too big to fill, or require the strength of steel, welding panels is the only option. If a long lasting repair is required, only butt welding should be used.

Rust proofing should be rechecked and if necessary retreated every two years.