Acid rain marks on a car, and what removes them
The spots people here call acid rain are almost always water marks: minerals left behind when rain or tap water dries on hot glass or paint. Fresh ones wash off. Older ones need a polish. The ones that have eaten into the surface can only be reduced.
On the windshield, taking them off is what our windshield buffing service does at your address. On the paint it is part of the exterior detail. The rest of this page helps you tell which kind you have before you buy anything.
Is it really acid rain?
Real acid rain exists. It is rain carrying sulfuric or nitric acid from air pollution, usually around pH 4.2 to 4.4, where ordinary rain sits at about 5.6 because carbon dioxide dissolves into it. Ordinary rain is slightly acidic everywhere.
What shows up on a car after a few weeks of the wet season is a different thing. A droplet lands on a warm panel, the water goes, and whatever it was carrying stays in a ring the shape of the drop. Dust and minerals from the rain, minerals from tap water if the car was rinsed and left to dry, and the residue from anything that dripped off a roof or a tree. That ring is what the shops sell acid rain remover for.
The name matters less than the depth. The useful question is whether the mark is sitting on the surface or has gone into it.
Three depths of mark
- Rings that sit on top
- Rain or tap water dried in the sun and left its minerals behind as chalky rings. They are still loose on the surface. A proper wash, or a spray detailer and a microfiber towel, takes them off. That is all they need.
- Rings that have set
- The same marks after weeks of wet and dry. Soap slides over them. On glass a glass polish removes them. On paint they need a polish that levels the clear coat around the mark, which is the same work as a fine scratch.
- Etching
- The mark has eaten a shallow dish into the surface. On paint a polish can usually take it down if it has not gone through the clear coat. On glass a polish takes the haze down and some pitting can stay, because glass has no clear coat to work into.
A quick test: wash a small patch properly and dry it with a clean towel. If the rings are gone, they were the first kind. If you can still see them with the glass or paint clean and dry, they have set or etched. For marks on paint that look like other damage, see what is on your paint.
If you buy an acid rain remover
- Read where the label says it can go. A Philippine-sold acid rain remover we checked lists glass, aluminium and stainless steel, and says plainly not to let it touch rubber or painted surfaces. That is a glass product, sold under a paint word.
- Keep it off the wiper blades and the rubber seal around the windshield, and rinse any that runs down onto the paint below the glass.
- Work on a cool panel in the shade. The same heat that baked the marks on dries a product before you can rinse it.
- If the label says nothing about paint, do not use it on paint. Marks on the hood and roof are a polish job, covered on our paint defects page.
Stopping them coming back
Marks form when water dries where it landed, so the fix is mostly about drying. A car rinsed and left in the sun will spot from whatever the water carried. On every wash we take the water off with a towel while the glass is still wet.
Wax on the paint makes rain bead and run off instead of sitting in place, which is part of why it goes on last in a detail. Once marks have started, catching them early keeps them in the first kind. Through a long wet season they move to the second, and a car parked under a dripping roof edge or a tree gets there fastest.
Book the glass or the paint at your address
If the marks are on the windshield and the windows, read what our windshield buffing service covers. If they are on the hood and roof, that is the interior and exterior detail. Say in the form how long the marks have been there and we will look before quoting the job.
Sources
- US Environmental Protection Agency, what is acid rain: sulfuric and nitric acid, normal rain at about pH 5.6, acid rain usually pH 4.2 to 4.4. epa.gov
- Mike Phillips, Autogeek Online, the three types of water spot: surface mineral deposits, etching, and how each is removed. autogeekonline.net
- Klassic Acid Rain Remover, product listing on PartsPro: glass, aluminium and stainless surfaces, not to contact rubber or painted surfaces. partspro.ph