Is it possible for fog to appear inside the windscreen glass?

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Car clinic: windscreen

Q. When the windscreen of our 2013 Volkswagen Touran fogs up, the wipers are able to clear the outside and a wipe with a leather cloth clears the inside, but there is a foggy area that clears only after the windscreen has been warmed for at least 15 minutes. It looks as if there is fogging inside the glass. Is that possible?
ES, Thurso, Highlands

A. The windscreen is constructed of a thin layer of plastic sandwiched between two plates of glass. It is possible for the glass to “delaminate”, meaning that one or both layers of glass separate from the layer of plastic between them. This makes the glass turn milky and difficult to see through. If this is the case, the only solution is a replacement windscreen.

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However, apart from being extremely rare, this condition would not disappear when the windscreen warms up, so it is much more likely caused by some stubborn dirt or grease on the screen. When the heater is on the demist setting it is blowing air from the road, including lots of traffic fumes, against the inside of your screen, creating a thin, oily film. On the outside, polish from car washing or grease thrown up from the road can form a hard-to-clean patch.

A thorough clean is likely to solve the problem. The best methods are traditional and cheap. For the inside of the glass, use a mix of spirit vinegar and water with a few drops (and only drops) of washing-up liquid. After a thorough rub all over, dry the glass with a clean towel or microfibre cloth.

For the outside, the best cleaner is methylated spirits. Put some on a clean cloth and rub it all over the windscreen — it will get rid of the water-resistant grease or silicone car polish and leave the glass clear.
Sunday Times Driving Car Clinic: Tim Shalcross, advance driving advice

Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motoring – read more from Tim here.

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