To remove deep hard water etching rings (ie. water spots), with a buffer I would use 3M's 'Microfinishing Compound' and then use 3M's 'Finesse It' finishing material to remove all scratches from the compound. This stuff works VERY well on clearcoat surfaces. Be sure to use (obviously) different buffing pads or cloth when using each material. Most automotive paint stores will carry both of these products...not cheap though - about $27.00 per bottle.
When hard water is allowed to air dry on painted surfaces, the limestone in the water will actually cut a tiny groove into the paint surface, much like a mircroscopic etching tool. And if your homes' water source has heavy amounts of calcium carbonate in the water (ie. limestone), then you should consider taking steps to rid your water supply of the problem (your plumbing system will thank you). And, guess what...I can help
I provide whole home water treatment systems for a living and could point you in the right direction to find a good local company in your area.
I live in an agricultural area with moderately hard water. Waters spots are the bane of my existence - they NEVER come off easily. However, there is an Amway glass cleaner in a white paste form that has always removed spots from glass - no matter how old. I would bet that it could help your paint as well. I would also bet that you probably know someone who sells Amway....doesn't everybody?
Years ago, I used a product called Liquid Ebony. It was designed to remove swirl marks and spider webs from the paint, and it did! When done, it looked like you could put your finger into the finish, and it didn't take a lot of the product to do an entire car. Haven't seen it lately, and I don't know if/how it would work on clearcoats. I just remember it was good stuff.
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