Re: well water leaving spots
The spots are not eating away at the paint, its mineral deposits that are being left on the paint once the water dries.
Most polishing will take those spots out...depending on how bad they are you may have to use a compound. I did a Toyota that had the same issue, used Menzerna 203 with a Griots Garage polisher. Once the spots are removed, coating the truck with a good sealant, like Collinite IW845, will help keep the spots down, but you'll almost always end up with them.
2005 F-250 CC SB White V10