ive used acid on cars to remove water spots .. real acid .. its something you have to be very careful with . you can fry a paint job with it and you can also ruin glass in a hurry .. i would only do it again with a waiver from the customer .. i had a bad experience .. i know how to do it but i would avoid it
The acid bath will only remove the mineral deposits, any etching must be removed mechanically by an abrasive compound. An acid bath on the cheap side would be to initially wash the car, then wet the area (half of the hood, one door panel, etc.), take a MF applicator and dunk into white distilled vinegar (you can use full strength without worry since there is water on the panel), gently agitate the entire area for about 60 seconds and rinse with water. Check your progress and repeat if needed (probably not). Once finished rewash the entire car.
I did this but used diluted vinegar and the process took longer, and two applications to the hood. However DavidB from Autopia, who wrote the article I read about doing this, told me that full strength can be used. To be safe test on an inconspicuous area first.
Meguiars also makes a product in their marine line to water spot removal that is supposed to work really well.
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I believe that you are talking about the post by Hawaiianelement... and he really did use acid. The thread is called Weekend Nighmare (or something like that). He did a great job on that car! You should take a look at the pics!