Why does a smeary film keep appearing on my windscreen even after I clean it?

Q. A smeary film keeps appearing on my VW Golf’s windscreen. I clean the car every two weeks with a jet washer, and I use VW screenwash, Autoglym glass polish and glass cleaner on a microfibre cloth. I even clean the wiper blades, but still the film returns. Why? SF, Plymouth

A. Detritus in the air such as diesel exhaust, tree sap, particles of tyre rubber and general grime accumulate on windscreens, no matter how meticulous your cleaning regime.

Where and for how long the car is driven will be a key factor: for example, sitting for hours a day in rush-hour traffic is far worse than pootling through the countryside.

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Check that you’re adding enough screenwash — the recommended concentration will be given on the bottle. If you don’t use it already, you might want to switch to Comma Xstream ( £15-£20 for five litres of concentrate), which often comes top in independent tests.

If the problem persists, give the screen a deep clean. A cheap option is to scrunch up newspaper and rub the dry screen with it.

To go a stage further, Glass Doctors (01494 813 583, glassdoctors.co.uk) suggests using a polish that includes cerium oxide to remove a microscopic layer of the glass, including the unwanted coating. Suppliers include Glass Polish Shop (0800 072 7857, glasspolishshop.com), which sells it as part of a selection of windscreen repair and polishing kits starting at £16. Its kits are designed to be used with an electric drill polisher because of the large area of glass involved. First, and this is crucial, test the process on a small area in the corner of the screen to get a feel for it, and be careful not to overdo it.

INSPECTOR GADGET
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory – read more from Dave here.

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