Originally Posted by DakarM

claybar wont remove oxidation or swirl marks. It will however do a very good job of cleaning the paint than any thing else short of full or abrasive 3 step polishing compound.

It took 8 hours because I clayed the whole car 3 times. On some panels with really caked on water spot, I had to hit it 3-4 times. The water was really hard and it just happened that day we hit 105F so it really got a chance to bake onto the paint

BTW you can also try using white vinegar. It should break down the minerals (the water spots). I did this once on the M Coupe and it removed most of the water spots. Just wash your car right after wards so it doesn't smell like vinegar.