You usually have your car washed thoroughly when prepping it for a coat of wax. After drying it, you sometimes have watermarks on the paint. Now, if you have your car washed at the corner car wash, they would usually tell you the only way to remove the watermarks is by rubbing it down.
Now, the stains can either be caused by improper prep before applying the car shampoo. If you spray water on a dusty car, you would notice the water sometimes has a hard time clinging on to the paint. The dust acts as a water repellant. And if this happens often, the dust could cling on to the paint and would be difficult to remove later on. An easy way to remove the dust is to use a soft rag and a hose wetting the area you are wiping, rinsing the rag every so often. I would suggest wiping the glass first then the body. This also gives you a feel of the condition of your paint.
If you notice some roughness at the bottom of your fenders or doors, that could be road grime. If your man enough, you could try using a rag dipped in kerosene. This is the old school way. But be warned the kerosene may react with the topcoat of your car. You could try using a bug and tar remover. They usually have this at the auto care products section of True Value or Ace Hardware. Note: please read the instructions carefully.
If you’re planning to use a degreaser, test it out on a small area first. There are some brands that are strong enough to harm the paint. If you are unsure, try and get in touch with the guys from Meguire’s, Adams, 3M, Turtle Wax or Mother’s. Forgive me for not being versed properly on their product line. If by chance you stumble on Mafra car products… they showed me one of their degreasers for removing stains (petroleum-based and tree sap) that is not harmful to car paint. It’s called “JEDI.” No, it is not the one used to keep Darth Vader’s helmet and personal T.I.E. fighters nice and shiny. About year ago, they were kind enough to drop by my place and offer me their products. This is one of their products that got my attention. True enough, it was not harmful to the paint finish, but it is strong enough to degrease your engine bay.
Turtle Wax had a product that was specific for road tar removal and I am not sure if it’s still on the market. And yes, it did what it was supposed to. Try using a clay bar. It’s supposed to lift dirt from the paint with out harming it. I have not used this myself but I have seen demos of this product and there is a difference after its use.
Another cause for those nasty watermarks is the water itself that you are using. There are some areas where in the mineral content of the water is high. This evident if you look at the bottom of your electric thermos. After awhile, you notice sediments at the bottom and it’s hard. Or you notice white stains on your bathroom tiles where the water usually soaks.
I have used rubbing compound to remove those watermarks but I could see the paint on the rag I am using. Honestly, up to now I still rely on the professional detailers to remove those stubborn watermarks. Oh, before I forget… avoid prepping your car under the sun or when the body is hot. When the car shampoo dries up, it could also leave nasty marks. Always wash your car when it is cool to the touch. Cooling it down with water is not recommended.