I painted the furniture on my C8 with Kylon Camo OD. I didn't use any primer at all. Since it's CA furniture, it has a rough, textured finish, so paint adheres well to it. The stock is an ABS JG stock from my HK416. I didn't rough that one up either. Just painted straight onto it. I sprayed on about 5 light coats. The gun's been used in 1 game, but handled and shot a lot at home, and the paint hasn't worn through anywhere yet, although I suspect it will eventually wear through on the stock. No big deal though, as then it just looks like it's a gun that's seen heavy field use, and I'm ok with that. If it gets bad, I can easily re-spray it.
Just a tip - when prepping to paint, clean the plastic parts with isopropyl alcohol. If you can get the 99% stuff from the drug store, even better than the regular 70%. It will clean off any dirt or oil and impurities from your skin. While you clean it and after cleaning, handle the parts with rubber gloves as to not get any oils and such on it.
Spray it in light coats, allowing about 20 mins to dry between coats. Apply 3-5 coats for a nice, durable finish. If you want to have the finish last longer, you can get some Krylon matte clear and spray on a couple of coats of that once the base colour is dry. Let the parts dry at least 24 hours before handling them and reinstalling them on the gun.