As paint films are exposed.to the elements, the following events take place.

1. Ultra violet radiation begins to degrade the binder molecules by breaking bonds between atoms.

2. As the binder molecules become smaller, film strength declines and moisture penetration increases.

3. With temperature cycling the moist paint film is first heated and then cooled. The accompanying expansions and contractions put a strain on the film which is poorly resisted, because the binder molecules are now smaller and less mechanically strong.

4. The weakened film is now subject to mechanical rub-off or water wash-off.

5. As the unpigmented surface film wears away the pigment particles become exposed to the elements. Continued degradation of the binder may actually set the pigment mechanically free, at which point the paint is said to be chalking.

6. On exposure to sunlight and moisture, many pigments begin to lose their colour. The paint becomes lighter - it fades.

7. The processes described above are accompanied by a gradual loss of plasticiser from the film, making it stiffer. The stiffening film is less able to resist the strain of thermal expansion and contraction. As a result, microcracks form which allow moisture to penetrate and exert a lifting pressure when it is warmed by an increase in the surrounding air temperature. The overall result is that the paint film gradually erodes away.

Paint manufacturers are well aware of the above processes and take great care in producing products which resist these processes. Production personnel often fail to realize the effects they have on the product's performance. eg. over baked films are excessively brittle due to premature loss of plasticizer and/or excessive crosslinking. Underbaked films are too soft and do not resist moisture penetration well. Similarly, excessive film thickness will lead to poor mechanical performance and low film thickness to poorer moisture resistance.

Weathered paints may result in:

Bronzing - Where a metallic like sheen is seen from pigments separating from the paint surface and breaking down. This may be overcome by mild polishing.

Chalking - Where paint is removed like chalk due to resin breakdown within the paint film surface. This again may be overcome by mild polishing.

Both bronzing and chalking can be brought on by using restoration compounds containing ammonia, eg. Brasso will'bring back the gloss, but after a month or two the ammonia (because it is alkaline) will break down the chemical bonds within the paint surface, resulting in chalking, water marking and poor gloss.

Similarly, detergents and soaps which are a mixture of sodium and potassium salts of long chain fatty acids produced by saponification with alkali will attack paint surfaces in strong or concentrated form, because of the excess alkali present. Steam washing equipment is particularly prone to this if set incorrectly.

Where high film thickness of paint is used, solvents must be allowed to leave the paint film and will do so less efficiently from high film weight. This results in the surface of the paint drying while the underlying material still retains solvents. The surface takes on the characteristics of a dry film and subsequent solvent loss causes the paint to shrink, causing increased tension in the surface layers.

This tension may with temperature change result in cracking (checking, crowfooting or alligatoring). It can happen with all paints but is particularly susceptible to lacquer dry systems. Once the cracking has occurred, it is not possible to bury the problem, the paint must come off.

External contamination where chemicals come into contact with paintwork and degrade the film. Various contaminants are known, but popular ones are: 1. rusting from iron filings, polishing embeds these further into the paint and oxalic acid may be required to dissolve the iron. Common sources of iron are from railway braking systems, fabricating operations and body shop operations, chimney out puts. 2. Ants and fly hatches. During late summer, male ants (drones) leave the nest to find other mates. They fly for one day and at night head for warmth radiating objects. Dark blue, dark red or black cars only seem to be affected. If the following day the sun comes out and dries up the ant, who's bite contains formic acid, which can etch the paint or erode through the whole paint system. Waxed cars are particularly v susceptible, because the ant becomes stuck in the wax. Flies hatching from lakes and streams can lay their eggs on paint finishes. These eggs are glued on and in hot weather can etch or corrode through colour coats.