A Driving Instructor's Blog

Over the years, I have updated this article several times as I have found progressively better solutions to the problem. I’ve now found pretty much the complete solution.

The problem of smeared windscreens in the rain has been driving me nuts ever since I started driving, but it became a major headache once I started teaching people to drive.Smearing windscreen in rain

I guess everyone has experienced it. You get a few spots of rain, and when the wipers wipe you get a mosaic pattern left behind for a few seconds. But in heavy rain it’s like someone poured chip fat on the screen and you just can’t see properly. I’ve had varying levels of success removing it – scrunched up newspaper (no good), sodium lauryl sulphate (not bad), various solvents (fair), Clearalex (quite good) – but things came to a head when my lease company replaced my last car. In rain you couldn’t see anything, and absolutely nothing would get rid of whatever it was on the windscreen. I was close to assuming that the glass must have been damaged in some way since it was a brand new car.

Where does it come from?

In normal circumstances, every vehicle that uses our roads leaves deposits behind. Some of it is dusty, some is gritty – but a lot of it is oily. That’s why when it rains after a period of dry weather we’re advised to take care, because the road can be very slippery as water and oil sit on top of the tarmac before the oil is eventually washed away. While it is fresh, road spray is obviously going to be a mixture of dirt, oil, and water, and when this gets on to your windscreen you start to get smears. Now, up to a point, your screen wash can deal with it (unless you just put water in your tank, in which case you’ll get smears all the time). Eventually, though, the oil seems to bond to the screen such that removing it is no longer easy.

Another source of windscreen contamination is, oddly enough, washing your car. Even if you don’t choose the waxing option in the car wash, the brushes will be contaminated with wax from people who did, and if you’ve ever noticed how a single fingerprint can be smeared across almost a whole mirror or windscreen unless you use something which lifts it off, you’ll realise that a little wax (or oil/grease) can go a very long way. Even if you hand wash your car, small amounts of wax and oil gets on to your rags and gets spread on the glass. It also gets on to the rubber of your wipers, so it is smeared back on as soon as you use them, even if you got it off the glass. People often forget that the wax (and oil) collects below the wipers, and if you don’t clean that area properly, the wipers dip into it like a pen dipped into ink and spread it across the glass again.

The particular problem with my lease car turned out to be the result of a manufacturing residue as far as I can tell.

How can you get it off?

Most detergents and surfactants will remove these normal deposits with varying degrees of success, though car wax is particularly stubborn. Even Fairy Liquid works up to a point. Some materials are much more powerful and are much more effective. You can buy powdered products like Clearalex, which you add to your windscreen washer fluid. Clearalex works quite well, but leaves terrible white residues when it dries (which I find very distracting). I have also had success with Sodium Lauryl Sulphate (SLS), which is an anionic surfactant used in many household products. SLS removes quite a lot of windscreen gunk, but it won’t touch wax and it leaves a slight residue when used at the required concentrations. I was very happy with SLS until my recent vehicle change, where I found that no matter how much I used it would not touch whatever was on the glass.

After a lot of research, I found a reference to sugar soap. I’d not heard about this before, but it is used by decorators and builders to remove grease and dirt from surfaces prior to painting. Sugar Soap - from Screwfix

Wikipedia describes it thus:

Sugar soap as typically found in Commonwealth countries is a cleaning material of variable composition sold for use on surfaces affected by greasy or tarry deposits which are not easily removed with routine domestic cleaning materials. When in dry powder form it looks like table sugar thus causing the name.

The solution is alkaline and its uses include cleaning paintwork in preparation for repainting.

It also turned out to be dirt cheap, at about £2 for nearly half a kilo from Screwfix. I bought some, made up a batch using information on the pack, and gave the windscreen a good scrub using it. I rinsed it and took the car out for a run in the rain.

Initially, I didn’t see any difference, but after a few wiper passes the glass was getting noticeably clearer. The sugar soap appeared to have softened whatever it was on the glass and it was gradually coming off. So when I got back home I made up another batch and soaked some rags in it, then left them covering the windscreen (including the bit at the bottom) for about half an hour. I also cleaned the blades with it. This time the windscreen was absolutely crystal clear.

Sugar soap was great, but being a solid there was still the problem of residues if you used it in your normal washer fluid. However, that might have been the end of it had I not started using a local hand car wash. I was intrigued by how easily they managed to remove every trace of dirt and oil from the car with only a power spray and some mysterious hand-pumped spray guns with various liquids in them, so while they were working I did a bit of snooping and noted the names on the various drums and containers lying around. The important one turned out to be “TFR” – which I discovered means “traffic film remover”. Already this was sounding quite exciting (well, it’s exciting if you’re a chemist) due to the way TFRs operate.

In a nutshell, a 1-2% TFR solution gets all the oil/wax film off a windscreen in one go. The same concentration in your wash bottle keeps it off. I bought my TFR from a company called JennyChem. They also supply the mysterious cherry-smelling shampoo the hand car washes use. I use a combination of sugar soap (soaking using rags) and TFR as required, and a 1% solution of TFR in 10% ethanol/water as my screenwash. I also use this solution in a hand spray bottle for spur-of-the-moment cleaning, and for doing the inside window surfaces.

As time has gone by, I have started using the TFR spray to clean my alloys and bodywork in between visits to the hand car wash when I have an upcoming test. It removes brake dust from alloys like all get out, as well as summer tree gum and bird crap (especially when the little sods have been eating blackberries and insist on sitting on the telephone wire right above my driveway).

I’m getting a lot of hits on this now I’ve mentioned TFR.

As a footnote, my hand car wash has just started using what I am assuming is a liquid wax (I haven’t yet identified it) that makes water bead very easily (and very impressively) when it rains. The problem is that it gets on the windscreen, and it is a sod to get off (two sugar soap treatments did it).

Does TFR damage the windscreen?


Does TFR damage paintwork?

If it is the non-caustic type, and if it is used at the manufacturer’s recommended concentration, no. But remember that TFR will remove any wax you have applied, so you will need to re-wax  after using it on painted surfaces. However, removing wax is exactly what you want if it’s on your windows.

Strongly caustic types – which are cheaper and harsher, and often used to shift several centimetres of crap off the undersides of lorries – could damage painted surfaces if used at high strengths and if left on for too long.

The stuff supplied by JennyChem (linked to earlier) is not strongly caustic as far as I am aware, and is specifically designed for use on cars.

Does TFR leave a residue?

The stuff I use doesn’t – well, no more than normal windscreen washer solution does. You’ve got to remember that when you use your windscreen washers, you’re doing it to remove dirt on the windscreen. That dirt is visible, so when you wash it off it will leave visible streaks outside the wiper area when it dries. It’s like when a bird drops a load on the screen – when you wipe it off there’s a good chance it will sit on the screen at the edge until you scrape it off by hand. There’s not much you can do about that.

Is there a non-chemical solution?

A reader (from Australia) wrote to me recently to tell me that he had had success removing that new-windscreen film using Cerium Oxide paste. You can buy it easily from various places (including Amazon) in various forms – powder, paste, or block – and it is specifically used for polishing glass. If you buy it, make sure you get the finest grade possible – ideally, one which is specifically sold for the intended purpose.

Can you put oil on the windscreen to prevent smearing?

Or, as it was put to find the blog, “can u put oil on wind screen 2 prfent rain”? NO. It will make it worse. It’s oil (and suchlike) you are trying to remove. Put it on deliberately and you could end up killing yourself – you won’t be able to see properly.

You can buy things like Rain-X, which are intended to make water bead up and roll off more easily, but those who use it often complain that it is patchy in coverage and leads to worse problems with smearing when the wipers pass over the glass, especially as it starts to wear off. I nearly tried this, once, but the risk of it causing more problems put me off. I’m not saying it doesn’t work, just that not all reports about it are as positive as the advertising is.