A Driving Instructor's Blog

Smearing windscreen in rainI’ve mentioned this in the smearing windscreens article, but we’re approaching that time of year where it gets wet and cold, and a lot of crap gets thrown on to your glass and builds up into a nasty film that doesn’t easily wash off.

I’m always amazed that some people only put water in their wash bottles (if they have anything in at all). I’m even more amazed that some of those using just water are ADIs – and my amazement maxes out when I hear them trying to justify it.

Water on its own does not have sufficient wetting properties to attack oil, wax, and grease. Even proper washer fluid can have problems, which is why you get that mosaic pattern left behind when you wipe in the wet. You need a good detergent to clean off oily deposits, and a small amount of alcohol to assist with wetting. Alcohol also functions as an antifreeze, so whereas using just water means you’re going to get a popsicle with the first frosts, a proper washer fluid will protect you to well below freezing.

A typical concentrated screewash that you dilute yourself is usually used at levels between 10% and 25% in water depending on the weather, and a 5L bottle costs about £5. Depending on whether you’re making a summer or winter mix, every 5L of diluted washer fluid therefore works out at between 50p and £1.25. If you use the concentrate neat – I’ll talk about that later – then obviously a batch costs the full £5.

In a bad winter, with lots of rain and slush, I will easily get through 5L of washer fluid each week. In a dry summer, I might use only a quarter of  that. Over a whole year, I would be paying around £30 for washer fluid if I made it from concentrate. It’s not really a lot of money, but it’s still an overhead.

Of course, you can buy ready-to-use washer fluid, where the dilution has already been done for you. If you find a suitable bargain, you can pick it up for between £2 and £3 for 5L, but the normal retail price is similar to that of a bottle of concentrate. It means that you’re paying between £1.50 and £5 extra for what you can essentially get out of a tap! A lot of people buy this ready-to-use stuff, but if I were to do so, it could cost me between £75 and £170 a year depending on how much I’d paid for it.

Also remember that retailers are sneaky and they put prices up in winter. At one time, for example, my local Makro was doing BOGOF on the concentrate I used to use – and it was a good price to start with. But they only did it during the summer when demand was low, and in winter the price doubled and there was no BOGOF. If you buy it, stock up when it’s cheap.

So, is it possible to make your own screen wash cheaper than this? Washer fluid needs to be able to do two things:

  • clean
  • not to freeze when it gets cold

It’s basically just a mixture of alcohol and water with a bit of detergent.

Alcohol – as ethanol – functions as an antifreeze and a wetting agent. The whole subject of freezing point depression in alcohol/water mixtures is a huge topic in physical chemistry, but the bottom line is that pure water freezes at 0°C, whereas adding different amounts of alcohol lowers the freezing point. A 10% ethanol/water mixture freezes at -4°C, a 20% mixture freezes at -9°C, and a 30% mixture freezes at -15°C. A typical commercial concentrate (£5 for 5L) which claims it freezes at -6°C when used neat must therefore contain 15% alcohol.

Alcohol is the most expensive ingredient in screenwash, and a 15% solution will have 750mls of ethanol in a 5L bottle of concentrate. Ethanol can be purchased for around £2 per litre, so a 15% concentrate will contain about £1.50 worth.

Whatever detergent you use has to be relatively non-foaming – you don’t want bubbles blowing down the street when you use it – and it has to be the kind that is actually going to attack the crud that get on your windscreen. This is another big chemistry subject, but to cut a long story short, Traffic Film Remover (TFR) is ideal. TFR gets anything off your car – tar, oil, mud, insects, bird crap, dead squirrels, that sort of thing. I get mine from JennyChem, and it costs £12.50 for 5L. However, you only need to use it at a concentration of between 1% and 2%, so 5L will go a long way – you need about 15p to 30p worth of TFR in your DIY concentrate. One 5L bottle of TFR will make up to 70 batches.

A batch of DIY concentrate will therefore cost around £1.70 versus £5 for commercial brands. Each batch of diluted washer fluid will cost you between 17p and 45p.

What if the temperature goes below -6°C?

When we had those two cold winters some years back, the first one caught me out, and my washer fluid froze – even though I’d followed the directions on the concentrate bottle. I’m a chemist, so the solution was simple: I nipped into a hardware store and bought a small bottle of methylated spirit, and poured that in. After half an hour or so, my wash bottle was defrosted. That was when I got the idea of making my own washer fluid – I would always know what temperature it was good down to.

So, the answer is to increase the amount of alcohol in your mix. Even using your DIY mix neat is going to be a lot cheaper than buying a low-temperature version – the price is proportional to the amount of alcohol in them. But for even lower temperatures, increase the alcohol to 20% or 25% in the concentrate and use it neat. It’s still cheaper than buying it.

How can I prepare for cold temperatures?

You just have to use common sense. In summer, using concentrated screenwash neat is just a waste of money, so it makes sense to use it at the 10% dilution (negligible antifreeze properties). When it gets colder, a 25% dilution will be good to about -2°C. If it gets very cold, use the “concentrate” neat to protect down to -6°C. If it gets really cold – below -8°C or so – add extra ethanol to the neat concentrate. An extra 500mls (£1 worth) will protect down to about -12°C.

Can I make it with more alcohol in it?

Yes, but be careful. Ethanol is flammable, and on its own has a flash point of 14°C (that means that at that temperature and above, a combustible vapour exists that can easily be ignited). A 10% solution in water has a flash point of 49°C, which is much safer. A 20% solution has a flash point of 36°C, which is still safe unless you store it in a very hot place. A 30% solution has a flash point of 29°C, and this is quite likely to be encountered in hot weather. My advice is not to exceed about 25% of ethanol.

If you make up the DIY concentrate using 1L of ethanol instead of 750mls, it’ll be good down to  -9°C. In fact, I am fairly certain that the brand I used to use was once safe down to that temperature, but somewhere between 2010 and the present it started referring to -6°C as the lower limit. I suspect they reformulated to cut costs, and dropped the alcohol content from 20% to 15%.

Don’t store a strong winter mix in your car during the summer. And definitely don’t carry any neat ethanol during the summer months, and only the smallest amount in winter in case you have to do an emergency defrost like I did that one time.

It seems complicated making your own

That’s why there is a market for ready-to-use screenwash. It’s up to you.

What kind of water do you use?

Tap water will work for most people. It’s probably what they use when they’re diluting commercial concentrate. However, tap water contains dissolved minerals – especially in hard water areas – and it leaves white streaks when it dries. It’s best if you use distilled or deionised water. Distilled/deionised water doesn’t do that. The only problem is that unless you already have access to it – and you might – buying such water is relatively expensive (50p per litre), and would add £2 to the cost of your DIY concentrate.

Personally, I used either rainwater (boiled) or the condensate from a dehumidifier we have to inhibit condensation and mould growth. If you store this source of water, make sure you store it with the alcohol already added, as that inhibits the growth of algae.

I just use water

Water on its own is no good. If the temperature falls, it WILL freeze. Even if it doesn’t freeze in your main washer bottle, it will in the pipes and at the nozzles. Water alone also doesn’t clean many things off the glass – it won’t touch oil, grease, or squashed insects, and it will struggle with tree sap.

If you do get a freeze up, trying to use the pump might cause it to burn out. Although I haven’t come across the problem recently, it can also cause pipes to burst or become detached from the main bottle (many years ago, that was a regular occurrence on a Citroen Xantia I used to have).

And remember that if you are driving without the ability to keep your windscreen clear, you are committing an offence. The Road Vehicles (Construction and Use) Regulations 1986 says:

Every wiper and washer fitted in accordance with this regulation shall at all times while a vehicle is being used on a road be maintained in efficient working order and be properly adjusted.

Arguably, you are not complying with this if you just use water. If it freezes (or the bottle is empty) and you drive, you’re definitely not complying with it. It is shocking that some ADIs are apparently doing this.

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