Car Clinic: Why numberplates are white and yellow, a terminal Mercedes sunroof, parking a 911 outside and repairing light scratches

The Car Clinic experts

THE DEALER
Jason Dawe is our used-car expert and has appeared on Top Gear and the Morning Show

INSPECTOR GADGET
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory.

LADY DRIVER
Emma Smith is a journalist specialising in consumer issues and is a regular Driving contributor

Q. Must numberplates be white at the front of the car and yellow at the rear? I have seen French cars with white plates on the back — they looked more attractive.

TS, Sheffield

A. All UK-registered vehicles manufactured after January 1, 1973, must display numberplates that are white at the front, yellow at the rear, have black characters and be made of reflective material.

Military vehicles use non-reflective black plates with white lettering because reflective plates could aid the targeting of laser-guided weapons. If you drive with incorrect plates you are unlikely to be attacked by laser-guided munitions, but you could be fined up to £1,000 and your car will probably fail its MoT.

We spoke to the Driver and Vehicle Licensing Agency (DVLA) and were told the reason for the two colours is so that drivers can tell which is the front or back of a vehicle simply by glancing at the numberplate, and therefore whether it is likely to be moving towards or away from them. Yellow was chosen because it provides the biggest colour contrast, after white, with the black lettering.

French vehicles used to have yellow plates at the back as well, until the registration system changed in 2009. Since then both front and rear plates have been white with black lettering. With a few exceptions, owners of foreign-registered cars living in Britain have six months before they must inform the DVLA and register their vehicle, which will mean a change to UK plates.

ES

Q. My wife’s 2001 Mercedes A 190 had a factory-fitted “lamella” sunroof. The original malfunctioned in 2010 and was replaced by a Mercedes specialist for £1,675. This replacement now refuses to work. The same specialist garage claims the sunroof cannot be repaired and must be replaced again. Is it correct?

BC, Stokeinteignhead, Devon

A. The car’s lamella sunroof was supplied by a company called Webasto, based near Munich. It is one of the world’s leading suppliers of sunroofs but unfortunately this design was not one of its finest — it seems your first one was unusual in lasting a whole nine years. The main problem seems to be with plastic gears in the mechanism, which wear down rather quickly and then refuse to turn.

Your specialist is not able to get spare parts from Webasto directly because the company is under contract to supply parts to Mercedes. And the car maker itself is not able to supply the plastic gears as separate parts, although some other components of the sunroof assembly are available (in case your problem lies elsewhere).

Your specialist could buy an entire replacement lamella sunroof from Mercedes, but this would probably cost more than the car is worth, and the lamella’s complicated design means you can’t swap it for something more conventional (and more affordable). The only financially sensible answer is to turn up the air-conditioning next summer and put up with the lack of a sunroof.

DP

Q. I am thinking of buying a six-year-old Porsche 911. I do not have a garage, so would the car cope with being kept outside on the drive? In addition, is two or four-wheel drive better, and what will be the running costs should I drive 5,000-6,000 miles a year?

MS, Giffnock, Renfrewshire

A. Being kept outside should be no more of a problem for a 911 than it is for any production car, provided you stick to a coupé rather than a cabriolet or a Targa (Porsche’s name for a semi-convertible with a large opening roof section), which are understandably more prone to leaks. However, if you leave the vehicle for extended periods, invest in a car cover.

When choosing between two and four-wheel drive versions, the main consideration should be how you intend to use the car. Some Porsche lovers insist on the “purity” of rear-wheel drive, though such cars can be tail-happy. But considering where you live, you may benefit from the 4×4’s better grip in wet and icy conditions.

As for maintenance costs, there is a good network of independent Porsche specialists, and even main dealers can be cheaper than you might expect (the standard charge for a minor service is £480, or £610 for a full service). Porsche owners’ clubs can be a good source of general advice: try porscheclubgb.com or the Independent Porsche Enthusiasts Club at tipec.net.

Expect to get between 20mpg and 25mpg in mixed motoring. Keep insurance premiums down with a limited-mileage policy — up to 6,000 miles a year, say.

The biggest cost of buying any car comes in the form of depreciation. A 2007 Porsche 911 Carrera 4 coupé was just over £66,000 new. Today it will cost about £30,000, and will depreciate by a further £12,000 or so in the next three years.

JD

Q. Taking my father’s advice to remove bird droppings from my car with “warm water and a sponge”, I inadvertently applied its rougher side. This removed the mess but left circular, scratched patches that are visible but not too deep. Do I need the bonnet resprayed?

NF, London

A. It was once the case that paintwork damage that could not be remedied by the gentle application of some T-Cut required a trip to a body shop. Now, though, the advent of “Smart” — small and medium area repair technique — has made it quicker and cheaper to fix small blemishes.

A Smart repairer will be able to come to your home or workplace and carry out a repair in an hour or two, at a cost of usually between £100 and £300. First the area is sanded until it is smooth, and a primer is sprayed on and briefly heated until dry. Then the area is sanded again before the paint and lacquer coatings are applied, and finally it is heated again. This is all done using tools from the back of a van. Smart companies such as ChipsAway (chipsaway.co.uk) and Revive! (revive-uk.com) have franchises around Britain.

The technique is effective enough to blend in with the rest of the bodywork when applied to fairly shallow scratches but works best on less visible areas such as the lower part of a door. That said, if the scratches really are as shallow as you say, it should be suitable for the bonnet. If not, and your car does need to go to a body shop, you’ll be looking at a bill of £400-£700.

In future, to remove bird droppings, dampen them first before attempting to wipe them off — and invest in a microfibre cloth to avoid any more nasty accidents with the sponge.

DP

Got a car problem?

Email your question to carclinic@sunday-times.co.uk, or write to Car Clinic, Driving, The Sunday Times, 3 Thomas More Square, London E98 1ST, with a daytime phone number, your address and as much detail about your car as possible. We can’t reply personally, so please don’t send original documents or SAEs. Advice is offered without legal responsibility.