Water spots and hard water: what your tap water leaves on the paint
By Ollie B - SoapX Ltd.
A water spot is not dirt. It is the chalky mineral that was dissolved in your tap water, left behind when the water dried. How bad it gets depends mostly on how hard your water is and how fast it dries.
The short version
- Hard water carries dissolved calcium and magnesium. When a drop dries, the mineral stays on the paint as a ring.
- The harder the water, the more it leaves. Very hard water leaves at least three times as much as soft water.
- Sun, wind and warm panels dry water faster, so the mineral is left before you can dry the car.
- Fresh spots wash off. Spots left to bake can etch the clear coat and then need polishing.
- The fix is simple: wash in the shade, rinse a panel at a time and dry the car straight away.
How much mineral is in your water
Water hardness is measured in milligrams of calcium carbonate per litre. The Drinking Water Inspectorate, which regulates drinking water in England and Wales, sorts it into five bands. The last column is simple arithmetic: it is roughly the weight of chalky mineral left behind if one litre of that water dries on the car.
| Band | Hardness in mg per litre | Left behind by 1 litre |
|---|---|---|
| Soft | Up to 100 | Up to 0.1 g |
| Slightly hard | 100 to 150 | 0.1 to 0.15 g |
| Moderately hard | 150 to 200 | 0.15 to 0.2 g |
| Hard | 200 to 300 | 0.2 to 0.3 g |
| Very hard | More than 300 | More than 0.3 g |
A third of a gram does not sound like much. Spread across a dark bonnet as hundreds of dried rings, it is very visible. Tap water also carries other dissolved salts on top of the hardness, so the real figure is a little higher.
How to check your own water
- Your water company publishes the hardness for your postcode. Search its website for water hardness or water quality.
- The Drinking Water Inspectorate publishes a map of typical hardness across England and Wales, and advises asking your water company for an accurate local figure.
- Hardness varies a lot by region. Bristol Water says all the water in its area is moderately hard to very hard. Scottish Water says most Scottish supplies are soft to slightly hard.
- There is no legal limit for hardness in UK drinking water, so a high figure is not a fault.
Why spots are worse in summer
The mineral is only left behind when the water evaporates. Anything that speeds up drying gives you less time to remove the water first.
- Direct sun and hot panels. A dark panel in the sun dries a rinse far quicker than one in the shade.
- Wind. Moving air dries beads quickly, even on a cool day.
- Beads that sit. A small bead has a lot of surface for its volume, so it dries fast and leaves a tight, dense ring.
We have not put a stopwatch on this, so we are not going to quote drying times. The practical rule is enough: if the panel feels warm to the back of your hand, treat it as a panel that will spot.
Two kinds of water spot
- Mineral sitting on the surface. It looks like a pale ring and you can often feel it. A proper wash usually removes it. Stubborn rings need a water spot remover, which is a mild acid that dissolves the mineral. Follow its label and rinse well.
- Etching. If mineral bakes on repeatedly, it can mark the clear coat itself. The ring stays after washing and you cannot feel it. That needs polishing with a product such as No.07 Polish and Paint Prep. Our polishing guide shows how to do it.
Acids need care on some finishes. Our pH and surface damage guide shows which surfaces can take them.
How to stop spots forming
- Pick your time. Wash in the shade, early or late in the day, on panels that are cool to the touch.
- Work a panel at a time in warm weather. Wash it, rinse it and move on, so nothing dries where you are not looking.
- Rinse with an open flow. A gentle stream of water with no spray nozzle sheets off the panel and leaves far fewer beads than a mist.
- Dry straight away. Start with the glass and the top panels. The 1400 GSM Twisted Loop Drying Towel lifts water in one pass. Our guide to drying without swirls covers the method.
- Keep protection on the paint. A sealed panel sheds water more easily, so there is less left to dry. No.08 Wax and Sealant takes minutes to apply.
- Think about your last rinse. In a very hard water area, a final rinse with filtered, deionised water leaves nothing behind when it dries. Rainwater from a butt is naturally soft and works for bucket rinsing too.
Hard water and foam
Hard water also makes every detergent foam less, which is one reason snow foam can look thin in some parts of the country. If that sounds familiar, check the rest of the setup with our foam cannon orifice calculator.
Quick answers
Are water spots caused by the shampoo?
Mostly no. The ring is mineral from the water. Shampoo that dries on the panel can add a film, which is another reason to rinse well and dry promptly.
Will water spots damage my paint?
Fresh ones will not. Spots that are left to bake on again and again can etch the clear coat, and then they need polishing out.
Is it safe to let the car air dry?
In a soft water area on a cool, still day you may get away with it. In a hard water area, air drying is how water spots are made.
Does deionised water really dry without spots?
Yes, if the filter is working. Deionised water has had its dissolved minerals removed, so there is nothing to leave behind.
Kit for this job
- Twisted Loop Drying Towel 1400 GSM
- No.05 Contact Shampoo
- No.07 Polish and Paint Prep
- No.08 Wax and Sealant
Where the figures come from
- Hardness bands and the note on legal limits: Drinking Water Inspectorate, water hardness.
- Scotland: Scottish Water hardness factsheet.
- Bristol: Bristol Water, water hardness.
More from the Journal
Always read the label. Directions, dilutions and safety advice for every product are here:
