This reply is brought to you courtesy of Autopia
The same water we use to bathe our cars can also damage our cars' paint. The spots and damage are caused by the minerals in the water. When water evaporates off of your car's paint, it leaves behind the trace elements it contains. Calcium and metals are the most damaging elements found in your tap water, whereas rainwater may contain damaging acids from air pollutants.
Avoiding water spots is easy if you chase after them. The best solution is to use a quick detailing spray after you wash, or as soon as you discover the spots (i.e., when your neighbor's sprinkler gets you).
If the spots are allowed to dry and bake on, they will attach to and harden on your paint. When this happens, you need to use a mild acid to get them loose. Believe it or not, the best acid is also the least expensive and most available: a gallon jug of distilled white vinegar.
Expert car detailers have known this secret for years. If you take your car to a pro, they will tell you about the "magic acid bath" and charge you $60 or more for the pleasure of smelling like a pickle. Save the $60, put on some gloves, and get to it.
To give your car the magic acid bath, first wash your car with your normal car shampoo, rinse, then use the distilled vinegar. Just wipe it on with a sponge, and rub it in. Do one section at a time, let it sit 30 to 60 seconds, then rinse. When you're done, wash the car again with shampoo and rinse. By the way, vinegar will remove your wax. So, be prepared to re-wax your car after the vinegar treatment.
Sprinklers showered this car with hard water. The car sat in the sun for several days causing the hard water deposits to harden. Vinegar will remove the spots from the aluminum bumper, plastic tail lights and rubber trim, but the paint will need to be polished and buffed with a buffer.
If water spots are allowed to stay for more than a week or so, the minerals will etch the paint. In this case, using vinegar will remove the mineral spots, but the paint will have etched spots (dimples). In this case, it is necessary to use a medium cut polish or fine cut polishing compound to restore the paint surface.
A buffer in the hands of a pro can do wonders on a car that has heavy oxidation or minor scratches. Most body shops can use a buffer to blend touch-ups to perfection. Unfortunately, many detail shops and buffer owners don't know how to use the tool, or they use the wrong buffing pads or compounds.
Swirl marks are nothing more than micro-marring in the paint surface. Under a microscope they appear to be a scratch; however, you cannot feel the scratch with your fingers or finger nail.
The reason swirl marks and other micro marring show up so prominently on black and other dark colors is because the sides of the marring reflect light. When you polish, the edges are rounded and reduce reflection.
This diagram shows what severe micro marring might look like in a cross section of paint.
Incorrect use of a buffer or polisher is not the only cause of swirl marks. Every time you wash or wipe down your car you create micro-marring. The severity of the micro-marring depends on your tools and the contaminants present. Here are the ten most frequent causes of micro-marring (swirl marks):
1. Polishers/buffers with the incorrect pad or an untrained operator
2. Harsh polishing compounds and paint cleaners
3. Towels and applicators containing polyester threads
4. A dirty chamois or a chamois that has not been properly maintained
5. Wiping down a dusty or dirty car with a dry towel
6. A dirty car duster or a car duster used on a car with too much dirt on the surface
7. Not keeping your wash mitt or sponge properly rinsed
8. Automated car washes with brushes and other wipers
9. Not rinsing your car completely before washing, or not washing your car thoroughly before drying
10. Using a car cover when the car or the cover is not clean
The most noticeable area for swirl marks is the hood and trunk. On dark-colored cars, they might also show up on the doors and fenders. Bright colors do not show swirl marks as well because they reflect more light. That doesn't mean they don't get swirl marks, they just show up more readily on dark colors. Black, of course, is the worst of all colors for displaying swirls and other imperfections.
This picture, taken by Scott Borders prior to detailing, clearly shows buffing marks and other micro marring created by an inexperienced dealership detailer with a buffer. No doubt the Ford truck looked great when the owner took it home, because most buffer jockeys use a high gloss wax that temporarily fills the buffing marks and other minor scratches.
After buffing and polishing, the swirl marks and hazing are no longer visible, and the paint surface has a deep, wet look. Scott did a great job, and the truck owner is very pleased.
Swirl marks can be removed by polishing. Polishing out swirl marks without a buffer is a lot of work. I recommend polishing your car one small section at a time so you can see the progress.
To remove swirl marks, use a good medium cut polish. Many polish manufacturers market a special polish for removing swirls. Swirl remover polish formulas typically contain fillers and oils to help hide swirl marks. Over time, with regular polishing, swirl marks will diminish. It's very difficult to completely remove swirl marks. Even the best towels and wash tools cause some micro-marring.
POLISHING TO REMOVE SPOTS & SWIRLS
If your water spots or swirl marks are severe, I recommend using a product like Einszett Ultra Paint Polish or 3M Perfect-It Fine Cut Rubbing Compound. Follow the application of a fine rubbing compound with a good polish, such as Einszett Paint Polish or Einszett Metallic PolishWax. Use a good foam applicator pad to apply compounds and polishes. Use the rubbing compounds by hand only. Compounding with a machine is not a good idea because it removes paint too quickly to control.
Again, to keep spots to a minimum, try using a detailing spray after you wash. Likewise, to keep swirl marks to a minimum, make sure you have good wash and dry tools, never wipe, rub or polish in circles (straight line motions only), and be very careful with the use of buffers and polishers. If you have repair work done on your car, tell the painter you want a warranty against swirl marks. This lets them know you're clued in to a quality job. Regular polishing is the best way to remove water spots and swirl marks. Choose a good polish, good tools, and be patient.