I just got the PTR receiver and am currently looking for a G3 parts kit. Anyone have any advice? I have been wondering what I should do to the inside of the reciever, just blue it or is there some other finish I should use?

Parkerize the entire rifle after you build it.

Build Tutorial

Found this looks like a step by step guide, check the HK section as well if you can find Jeff (Ghilliebear2003 or it might just be Ghilliebear on here) he built my HK33 clone and has always been extraordinarilly helpful and insightful. As for the finish, most og the ones I've seen are more or less painted and baked, with only a few parts being parkerized. Pretty much depends on how you want it to come out looking.