Sir, Slopemeno is right, tape off areas that you do not want to touch.
The synthetic stocks are easy to sand and 600 will work to remove any seam or molding blemish. now if you do not plan on painting the stock. Use 1000 or 1200 wet sanding paper to put final finish on the stock if you are looking for a real smooth cheek hold. I have had good luck with the Dura Paints also. They are easy to apply with an air brush and will last a long time. They have all different colors, Any more I like to paint my custom made rifles with it unless someone wants the rifle parkerized or something else for a finish.
Good luck with your project
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This year will go down in history. For the first time, a civilized nation has full gun registration. Our streets will be safer, our police more efficient, and the world will follow our lead into the future!" -Adolf Hitler,1935"