Nylon 12 is pretty waterproof on it's own, but it can be finished in a number of ways, depending on how you will use it. We run a 3D printing business specializing in 3D printing Nylon 12 (www.tempus3d.com) and we have found that you can paint the parts yourself if they are not going to be used in a high-wear environment (for a cheap option). you can also send teh parts to a 3D printing service to have them vapor-fused (a chemical process that smooths and seals the outside) or fins a place that does a more industrial paint finish like Cerakote.

Good luck with your project!