You'll have to sane it to get any shine off of it so the paint will stick to it. Then you'll have to primer it, then paint it. Make sure it's what you want to do and I hope you like it when you're done. Cause it's gonna be a pain in the you know what to have to take it all back off. lol
Wouldn't it be easier to just buy the flat black stock in polymer or whatever it's made of? Less of a headache I'd think. Then you'll get the front piece and gas tube as well.
Your firearm, your choice. Good luck.
__________________ "My next door neighbors two dogs have created more shovel ready jobs then Obama has." - Gary Johnson
Depending on what condition you stock is in, it can be as simple cleaning it, sanding it, and painting it. This is actually not a bad instructional site on it