I've not done any synthetic stocks yet but I have painted a lot of automobiles & lately they have more plastic on them. The main thing with top coating plastics is adhesion. The compounds have oils & silicone type chemicals in them because they are flexible it is harder to get good adhesion. Go to an auto body supply shop or parts store that carries paint & tell them what you're doing & get some adhesion promoter &/or plastic primer & follow label info for applying & you should be able to get the durability you want.