Just an update to my original post. I've bought and used the followings and they all work great, much better than the P21S Total Auto Wash. As mentioned, this is for those stubborn 1-2 mm black dots/grime, from tar/soot/oil/etc. seen on lower panels and the back of the car. (Those with dark color cars, consider yourself lucky!)
1. 3M Wax & Adhesive Remover (from Atomic)
http://solutions.3m.com/wps/portal/3M/en...RglP6P3KR33VGbl
Work very well. No damage to the clear bra,clear coat, or paint from my observation and from reading the forums.
2. Mineral spirits. Ditto: Work very well. No damage to the clear bra or paint. The 3M stuff seems to work better but could entirely be my imagination.
3. Meguiars Scratch X
http://www.meguiars.com/?auto-paint-cleaners/ScratchX
The definitive solution since it removes the most stubborn of spots. Again, no damage to paint or clear coat.
4. Rejex. I applied Rejex to the left side of my car's rims 2 months ago and used the right side as control. I could confirm that Rejex works as advertised. When it comes time to wash the car, there is visibly much less dirt/grime on the left side, particularly the front rim, which tends to accumulate more dirty stuffs.