For you guys actually sanding the headlights...to avoid fine scratches or marks left over make sure you do
-600grit-sand horizontally
-800grit-sand vertically(until you see no more horizontal scratches)
-wet sand 1000grit-sand horizontally(until you see no more vertical sratches)
-wet sand 1500grit-sand vertically(until you see no more horizontal scratches)
-wet sand 2000grit-sand horizontally(until you see no more vertical sratches)
-wet sand 2500-3000grit-sand vertically(until you see no more horizontal scratches)
-polish in both directions and check again. I've done the above to about 5-6 pairs of headlights in the past 4 months and all have turned pretty much perfect with no issues.
-Wash them with soap/dish detergent
-Dry and apply some wax
As for them turning yellow soon after, it hasn't happend to any of the ones i've done yet, as long as you keep them clean.
I've done the above with the 3M set as well, and if you just use the rotary you'll still get some fine scratches in the end, more like fine circles...make sure you go over them by hand as well in opposite directions everytime.