The windshield has a large beaded edge. The portion of the bead on the outside is in the path of high speed laminar airflow. It has to go.
The bead outer part is removed with an air grinder. This leaves a rough surface. The surface is smoothed with a bastard cut mill file. The double cut bastard teeth leave a distinctive scratchy pattern on the plastic. The scratches are removed with a fine single cut mill file. The single cut filing is done at a 90 degree angle to the bastard filing, and it is done until the double cut filing scratches are gone.
Sanding is next. The sanding is done with 220 grit sandpaper at a 90 degree angle to the single cut file marks. All file marks are removed. Then, the sanding is repeated with 280 grit in a 90 degree angle to the previous coarser grit sanding. All coarser sanding marks are removed. This process is repeated through these grit sizes: 220, 280, 320, 400, and 600. I use 220 grit carpenter's paper, and for the finer sizes, K&S Engineeering "Flex-i-Grit" sanding film. The fine stuff is available at the local hobby shop. I always wash and wipe all grit off the windshield and my hands before I start with the next size finer paper.
Now I use a cloth mop wheel loaded with black emery. I make one pass buffing in one direction, then another pass buffing at a 90 degree angle to the previous pass. This is done until the sanding marks are gone. It takes a long time. Now I switch to a finer mop and repeat. One pass in one direction and another at 90 degrees to the previous. Two passes per grade for those finer than black emery. I use these in order, from coarse to fine: black emery, white rouge, tripoli, green rouge, red rouge, and blue rouge.
Sears carries the emery and rouge sticks, along with the mop wheels. I use a separate mop for each grade. The mops and sticks are stored in individual sealed bags so they do not pick up abrasive grinder dust. I wash my hands and the windshield when I switch grades. This hygiene is very important. Scratches from stray coarser particles can ruin a job and much rework is needed.
Finally, the windshield is polished with Meguiar's Mirror Glaze plastic cleaner followed by Mirror Glaze plastic polish. I buy this at the airport. The repaired area looks better than the rest of the windshield. Job done.