Meguiars clay bar, and their Quik Detailer product or any "quick detail" spray on product. (in between wax detailer)

Good Luck! Those spots are a b!tch to remove. I've used a light abrasive pad (kitchen cleaner sponge pad) some Armor All wheel cleaner, and some good 'ole elbow grease to remove the spots.

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-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi