If you have hard water spots, that's a different animal than the specks of industrial fallout, brake dust, and iron shavings you would clay bar out of your paint. The scale from hard water is from minerals and lime from wells and water pipes - however snow doesn't have those.
Here's a remedy list depending on whats up with your paint:
- Vinegar will remove hard water deposits, not good to use above the window level because it smells if it gets in the vents.
- Tree pitch and sap will melt with 99% rubbing alcohol (or everclear). Just soak a paper towel and rub for a few seconds, and quickly wipe away all the sap/alcohol goo with a clean towel before it dries back on. Alcohol also strips off old wax that may have dirt embedded in it.
-For road tar (those little asphalt blobs stuck on the bottom rocker panels), use a pre-paint solvent such as SEM Solv or PPG Acryli-clear, what would be a near impossible job becomes a 5 second wipe on, wipe off.
-Clay bar will work on the spots of metallic dust embedded in the paint. These are actual bits of iron stuck in the surface of the paint that then rust. The clay bar pulls them out like you would pull out a sliver (and leaves a hole in the paint, Meg's 'polymer sealant' as a wax can fill it in again).
Now you have CLEAN paint, then you can get out the buffer to rectify scratched-up paint problems and not just push dirt around and scratch the paint more.
-'advice from a white car owner'
Polishing a white car is like peeing in dark pants, you get a warm satisfied feeling, but nobody really notices.