Krylon Fusion is supposed to have superior properties for adhering to plastics. I agree with elkhtr about the prep of the surface. Roughing it up gives the paint mechanical adhesion. Scotchbrite pads are a good flexible abrasive of the right courseness.
I used the above paints to paint an HS precision stock once.
I did it very textbook. Took me a week. I did light coats with the paint once a day after work.
I found that it was still very prone to chipping however was easy to touch up with more paint.
Since then I have gotten into epoxy paint airbrushing with products like Armacoat, duracoat, cerakoat and the like. While they tend to be more expensive and require more specialized equipment, the finish is far far far more resilient and professional.
Ok as for prep. Synthetics tend to all be rough enough for paint to adhere well. The next is to tape off any parts that you don't want painted such as the barrel/action channel. Then after that you want to completely ensure that all oils/greases,etc are cleaned off the stock and that any cleaning you do does not leave any residue.
A good brake cleaner tends to be popular as they flash off with no residue. I also found that rubbing alcohol pads also work well as it flashes off, but that doesn't cut grease like brake cleaner.
I hung the stock and sprayed it with brake cleaner, let the excess drip off then spray it with compressed air until all of the cleaner has dried or flashed off, then went ahead with the painting.
The trick though is not to think you can paint the entire stock fully in a single session. Use a light coats, don't worry if there are light patches unpainted or very very thin levels of paint. Let that paint dry then do another pass, and again and again. If you pile on too much paint at once it will end up looking flat and chunky and will tend to chip.
