I have seen products that use the 3d printed part as a mold to make a part. Cast the part into silicone then use another plastic to cast from it. (see
http://justindunham.net/2011/08/maki...printer-parts/ )
You can coat the plastic in resin, possibly with some reinforcing such as carbon fiber, fiberglass or microspheres, but I would be sure (And test) the plastic wont react to the resins. Also if you don't do it neatly it will show...nothing some sanding and a coat of paint wont cover hopefully