A Driving Instructor's Blog

Smearing windscreen in rainPlease note that screen wash – even at its most concentrated – has a very low alcohol content and cannot be used as a hand sanitizer.

I’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.

It amazes me that some people – even driving instructors – only put water in their wash bottles (if they have anything in at all). And hearing them try to justify it just cracks me up.

Water on its own does not have sufficient wetting properties to attack oil, wax, and grease, and even proper washer fluid can have problems – it’s 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 as long as you have it at the right concentration.

You can buy two types in the stores – concentrated, or ready-to-use. In most cases, the ‘concentrated’ stuff will act as an antifreeze when used neat down to about -9°C for the most expensive brands, or -6°C for the routine stuff (some brands claim -20°C). The freezing temperature is dependent on the amount of alcohol in it, and it’s obviously cheaper to make with less alcohol. For most of the year, you might use this concentrated stuff diluted between about 1:5 to 1:10 with water, but the colder it gets the more concentrated liquid you need to avoid freeze ups.

The ready-to-use stuff is used neat, but you need to be aware of what temperature it will go down to before it freezes. Some brands are good to -4°C, and with the weather in early 2021 in the UK that would almost certainly freeze up on you. If you’re somewhere where it gets really cold, it would be no good at all. They also sell ‘summer screen wash’, which contains little or no alcohol.

The price of typical concentrated screen wash varies from about £5 per 5L in summer, to about £8 in winter (when you need it the most). The ready-to-use stuff is similarly priced, even though it is more dilute. In a bad winter, with lots of rain and slush, I can easily get through 5L of washer fluid each week. I use less in summer, but over a year it can mount up. Not to a huge amount, but it’s still an overhead.

If you’re going to buy it, my advice is to stock up in summer when the prices are lower, and only get the concentrate so you’re not paying someone to dilute it for you. You often get BOGOF offers in summer.

However, it can be cheaper to make your own (it definitely was when I first published this). I got the idea when I had a freeze up one time (I was late switching to my winter mix), and solved the immediate problem by nipping into a hardware store and buying a bottle of methylated spirits. Adding that to my wash bottle depressed the freezing point and I was running again within 30 minutes. There was also the fact that my garage was overflowing with the stuff I’d stocked up on.

When I started making my own concentrate I was using bio-ethanol, which is a clean-burning fuel for home heaters. However, most of this comes from the EU (even the UK-branded stuff), and as a result of the insanity of Brexit the price has gone up to cover import duties.  You can still get it for as little as £3.40 per litre, but the number of suppliers has dropped and the price of UK supplies has gone up. Alternatively, denatured ethanol supplied as a cleaning agent can also be used, and you can get it for as little as £3.80 a litre.

Washer fluid needs to do two things:

  • clean
  • not freeze when it gets cold

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

The alcohol – usually 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 alcohol lowers (depresses) 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 might claim that it freezes at -6°C when used neat, and this means it must contain 15% alcohol.

Alcohol is the most expensive ingredient in screen wash, and 5L of a 15% solution will have 750mls of ethanol in it. The cost of alcohol varies depending on current circumstances, but it’s cheaper the more you buy.

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 gets 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, who also supply a range of car products the car washes use. You only need to use it at a concentration of between 1% and 2%, so one 5L container goes a long way, and will make up to 70 batches of screen wash.

Finally, there’s the water. It depends on how anally retentive you are on the subject (for me – very). Tap water is what most people would use, but – and depending on where you live – this can leave mineral deposits on the glass as streaks if you’re in a medium or hard water area. You can buy deionised water, which has the minerals removed, but it costs money – unless you have access to a supply of it, which you might. Alternatively, rain water (boiled and filtered), or – and what I use – the condensate from a dehumidifier, provides soft water which leaves no streaks.

Making your concentrate is easy. Get an empty 5L container (the kind screen wash usually comes in), add 750mls ethanol, 75-100mls TFR, and top up to 5L with water. Mix well by shaking the container. Used neat, this will protect down to about -6°C, but in summer you can dilute it as low as 1 part to 5 parts of water (1:5).

Personally, I make my screen wash fluid ready-to-use as I need it (I make three or four batches at a time and just keep them on hand, making more as required). In summer, I just make it with less alcohol – 100mls or so – and use more water.

For comparison, if I bought a 5L bottle of screen wash concentrate right now (February 2021) which was good down to -6°C when used neat, it would cost somewhere between £8 and £11. A 5L batch of my own stuff good down to the same temperature would cost £3.12.

A 5L bottle of ready-to-use summer mix would cost £6 bought online. My own summer mix costs £1.22 (though it could be as little as £0.27 without any alcohol in it).

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What if the temperature goes below -6°C?

You just need a higher alcohol content. Protection to -6°C requires about 15% alcohol, but 20% will give -9°C, and 25% will give about -12°C. However, bear in mind the flash point of alcohol solutions. My advice is  not to exceed 25% alcohol by volume.

How can I prepare for cold temperatures?

Use common sense. In summer, a high alcohol content of the screen wash in your car is just a waste of money. Dilute the concentrate about 1:5 with water (it would freeze at just below -0°C). When it gets colder, and sub-zero temperatures are likely, a 1:1 dilution will cover you to about -2°C, a 2:1 dilution to about -4°C, and a 3:1 dilution to about -5°C. As we have said, the concentrate used neat would be good as low as -6°C.

Can I make it with more alcohol in it?

Yes, but be careful. Ethanol is flammable, even in water mixtures. On its own it 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.

A concentrate made using 1L (20%) of ethanol instead of 750mls will be good down to  -9°C. A 25% mixture will cover you down to -12°C. Any more than that, and be careful. 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.

Can I use isopropanol instead?

Also known a Propan-2-ol, 2-Propanol, and Isopropyl Alcohol (IPA).

Short answer, yes – but only if the sub-zero temperatures are a few degrees below zero. IPA has a lower flashpoint than ethanol, and  anything above 20% is risky. IPA also has a distinctive smell.

Can I use Methanol?

I’m just going to say no. It’s poisonous, and could be dangerous, so for that reason you should not use it.

Can I use methylated spirits?

Usually, this contains methanol as the denaturant – though sometimes other chemicals are used. It also has a strong smell. Apart from the time I used it in an emergency, I would advise against it. However, if you can find ‘denatured ethanol’ or ‘denatured ethyl alcohol’, and can be sure it doesn’t have methanol in it, that would be fine. It’s usually (not always) the blue stuff that contains methanol.

It seems complicated making your own

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

I just use water as a screenwash

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, and freezing water is quite capable of splitting pipes or closed containers. Water alone 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, even if it doesn’t split your feed pipes it can cause them to become detached inside the car (it was a regular occurrence (well, it happened twice) on a Citroen Xantia I used to have many years ago).

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.

Can you dilute ready to use screenwash?

Of course you can. It’s not a magic potion – just a mixture of water, alcohol, and detergent. I wouldn’t dilute the ready-to-use stuff more than about 50:50 with water, though, because the detergent probably wouldn’t do its job properly. And if it has a stated freezing point, just remember that diluting it means it will freeze at a higher temperature, and that could catch you out in winter.

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