One thing to keep in mind is the Plexiglas is really a lens.  And removing the scratch requires almost a "body work" approach.  You don't want to end up with a nice, rounded off, smooth, shiny scratch.  A deep scratch requires you feather the area around it, the deeper the scratch the larger the area.  Since most scratches are on the outside, the convex side you can use a DA or orbital sander, even by hand with a sanding block.  Just avoid the temptation to dig down into the scratch with a small sanding tool.  Bottom line, to preserve the optical quality of the windshield you need to work a large area if you have a deep scratch, the deeper the scratch the larger the area.

 

I got a fairly deep scratch on the inside of my windshield (don't ask) and used 1500, 2000, rubbing compound then the 3 stage Plexiglas polish system and it worked fine.  But I can still notice slight flaws in the optical quality of the windshield in that area.  It was difficult to work above the glareshield, by hand, and get it sanded out thoroughly.

 

Mike Easley

Colorado Springs