Water Spots Etching Your Paint…Why?

Water Spots, Acid Rain, Paint

What the Heck is Going on in the Rain?

Rainwater is not pure. It is naturally acidic, the result of water collecting on dust and smoke particles in the clouds. Comprised of water vapor and any number of contaminants like pollen, mold, bacteria, dust, soil, pollutants, plant parts, algae, and insect parts, these tiny particles make up the condensation nuclei. If you park your car or truck underneath trees or near foliage that drip rainwater onto the vehicle, add toxic chemicals to the impurity stew.

Acid rain in the form of raindrops, snow, and even the moisture that makes up fog, are on the more serious end of the water spot spectrum. If you live near an industrial center, chemical plant, or in the foothills of an active volcano, the contaminants that make up its condensation nuclei are more likely to be a chemical mix of minerals, debris, gaseous solids, and even radioactive material.

These contaminants are ravenous for your paint and glass. Usually delivered as dusty gray deposits that have bonded to the paint surface, hard water spots aren’t easily wiped away; and if left unattended, will “etch” the paint or clear coat. Damage from etching appears as rough, crescent- or circular-shaped marks where the raindrops evaporated.

The Two Types of Water Spots

Most professionals agree there are two types of water spots — Stage One (Type I) corrosion known as Above Surface Mineral Deposits (ASMD); and Stage Two (Type II) corrosion known as Below Surface Water Spot Etchings (BSWSE), damage caused primarily by “hard water” and acid rain. If left untreated, both threaten your glass and paint surface.

There are several products on the market made specifically for cleaning Type I water spots without scratching. Type I water spots can likely be removed with a simple paint cleaner and an applicator. The chemicals of the paint cleaner will soften the contaminants on the surface while the applicator will provide the light abrasion to remove them from the surface. In this case you’ll unlikely need to clay or polish so reapply your sealant of choice and you’re good to go.

Due to large deposits of calcium, magnesium, or iron, Type II BSWSE “hard water” spots and acid rain are the hardest to remove.  They may even sometimes take multiple attempts to correct the damage. In a Type II water spot, the chemical composition of whatever was in the water either was there long enough, or was powerful enough, to eat the paint! This is going to require a clay and buff regimen. With your clay bar and lubricant, go over the surface of the vehicle to remove all contaminants from the surface (including those not caused by the water spots). Once the surface is free of contaminants, starting with a less aggressive polish and machine pad would be best to see how effective it is. If on one panel you notice the etching still there, move to a more aggressive polish. Once the correction is complete, wash and seal the car again.


Even some of the most durable sealants on the market right now may not be enough to stop acid rain etching into the paint given enough time. However, it sure as heck isn’t going to hurt.

Where you live is going to determine how often you should be reapplying your sealant. For someone in Southern California, once or twice a year of a good durable sealant is plenty. However, someone in Hawaii may need three or four applications a year to protect from their harsh environment. This will provide a barrier where otherwise would leave your paint finish exponentially more vulnerable.

Like I said before, if a car is sitting outside for weeks in the hot sun with acid rain baking into the paint, a sealant won’t be enough to protect the paint. A regular car wash regimen is required (every 2-4 weeks), regular sealant application (1-4 times a year), and proactive care for the car is expected. If you don’t want water spots, make some room in the storage unit you call a garage to fit the car. If parking the car inside is not an option, then when it rains, within the next couple days, get out there with a simple detail spray and microfiber towel or a full on car wash to rid the surface of the harsh chemicals of the rain. This step is more necessary for people who see acid rain than ones who don’t.

For many of us, a sealant is going to protect just fine until we get into our next car wash. All the more reason to treat yourself and your car.