Smaller scratches can be minimized by first washing the letter, then applying car wax and buffing the scratch with a soft cloth.
Deeper scratches can be removed by wet sanding with extra fine grit sandpaper. Sand the scratch, wipe dry with a soft cloth and apply a liquid paste such as Finesse-it to the scratch. Buff out the scratch with an auto paint repair buffing wheel. Deep scratches that cannot be buffed out will have to be sanded with 1200 grit paper, then re-painted.