Smurkenstein how often are you using a mechanical polisher? And is the DA a rotary or orbital? I nearly went down that route when I my old Peugeot but sold the car before I needed it. It's certainly a technique I would like to learn - even if it is scary when you understand the potential damage that can be done!
Another consideration is water hardness. I have a water softener installed (I would never be without it now for more reasons than just car cleaning!) but that helps eliminate those watermarks that Smurkenstein mentioned. My neighbour uses rainwater from the water-butt and finds that to be very soft in comparison to local Cotswold hard water, but I often wonder what chemicals have washed down out of the atmosphere with it.
The megs polisher is DA=dual action (I think therfore it is orbital) - these are the safest type - not too aggressive. I typically use it once a year or when there is a specific defect that needs removing. No prblems so far. I have polished the whole car perhaps three times myself and had it done professionally twice. Even if you dont want to polish the car, its worth the money just to take out surface defects, scuffs, stains, etc.
I have really bad problems with water marks. I live in France and it comes from Aquifers under the ground and it is packed with salts. If you drink a pint of the untreated stuff it will trigger a bowel movement. The drinking stuff goes through reverse osmosis followed charcoal filters, but the throughput is too slow for rinsing the car (and its also hellishly expensive stuff).
