Stuttgart  - Rust has been the curse of car owners for decades and until protection from it was improved in recent years, many an owner could only despair at vehicles which literally crumbled away beneath them.

Anti-corrosion treatment has vastly improved, but according to Germany's car testing experts, modern vehicles are prone to rust too, as not every manufacturer takes the problem seriously enough.

Compared to between 10 and 20 years ago, the overall number of cars showing serious rust problems has declined considerably, said Hermann Schenk from the GTÜ technical examination organization based in Stuttgart.

Improved coating techniques along with higher-quality steel ensure that key components such as frame cross-members are less likely to disintegrate into brown powder. Around 75 per cent of steel in most modern vehicles has been galvanised with zinc.

Rust is caused when bare metal reacts to moisture by oxidising, leaving a fine reddish-brown coating which can puff up and crack if not treated in time.

Rust nevertheless refuses to go away, said GTÜ-Expert Schenk.

Experts frequently encounter examples of carelessness on behalf of the manufacturers who have skimped on rust protection. Arnulf Thiemel from Germany's huge ADAC motoring organization says the same thing: "Rust is unfortunately still an issue".

Thiemel blamed many rust problems on design faults and component defects as well as the misguided view of some makers that saving on anti-corrosion measures is a good way of keeping prices low in a highly-competitive market.

The ADAC maintains no detailed figures on the rust menace but receives plenty of feedback from members: "There are complaints about vehicles which you would not expect to be affected," said Thiemel.

Ford's Mondeo salon was notorious for catching the rust bug along the door edges, and Mazda 6 models were apt to go rotten around the wheel arches. Even the Mercedes E-Class hit the headlines with corrosion problems along with the company's A and B-Class models.

According to Thiemel, the anti-rust compound was apparently applied too thinly by the factory to be of any use.

Rust problems still crop up regularly during the testing procedure for cars - under German law vehicles are subject to a stringent safety test every two years. Among the cars aged between three and five years old examined by the GTÜ, 3.6 per cent were found to have chassis, frame or bodywork defects, of which 2.5 per cent were caused by rust.

Among older cars up to seven years old, the overall figure was 4.1 per cent - 3.1 per cent of them succumbing to rust.

For cars aged over nine years, the figures were 13.6 per cent and 11.6 per cent respectively.

According to Schenk, modern testing procedures do not always highlight rust problems. Most cars rust from the inside out. Water collects in components such as the frame, floor and bottom of doors from where it is unable to escape.

The growing moisture and condensation then leads to rust which eats its way through to the outside. Many modern cars also feature plastic planking or cladding which can conceal rusty bodywork.

Testers in Germany are not obliged to start dismantling a car in order to find these hidden rust traps, and as Schenk points out, many car-proud customers do not take kindly to the time-honoured method of tapping the bodywork with a hammer to detect weak spots - a tinny, metallic sound indicates sound bodywork while softer areas, where rust has already taken hold, give off a duller and denser sound.

Regular trips to the car wash can help prevent rust gaining hold -especially in the winter months when the roads are strewn with potentially-corrosive salt. Damage to bodywork or paintwork should always be swiftly repaired, said the expert.

"By not doing anything you are giving rust a chance. No amount of zinc-plating can help then." (dpa)