Sandpaper is great for finishing lots of things and optical surfaces are not one of them. The shield was sanded with 220 grit to take out as many blemishes as I could detect. Then I moved onto 320, 400, 600, 800, and 1000 grits. Each was sanded at right angles to the sanding direction of the coarser grit. The sanding was done until all scratches from the previous grit were removed. This was easy to see. The coarser grit scratches were across the direction of sanding for the finer grit.
The Micro-mesh pads are a lot better for polishing out the plastic. The stock removal must be done before they are used. They are good at smoothing out surface scratches - not leveling. The pads are sold by Rockler Woodworking, a national chain. Mine were bought at their store in Beaverton. The Micro-mesh grits I used were 3200, 3600, 4000, 6000, 8000, and 12000. The 12000 grit leaves a finish surface with adequate optical quality. No further polishing was done.
It took 4 hours of wet sanding per grit with 4 more hours each for the 220 and 3200 grits. That is 52 hours sanding for one little bike windshield. This should be factored into the time estimate for making a shield with this method. A big car windshield would be a bit much.