Depends how durable you want the finish to be.
In a perfect world you would media blast the parts before refinishing them but that may be over kill in your application.
In my own recent 10/22 build I stripped down the barrel/receiver; glass bead blasted it, then used a HVLP spray gun to coat it with Arma-Coat. It took a fair bit of time and money but looks good. However with just some sand paper, elbow grease and cheap rattle can spray bomb you can get 80% the same results for 10% investment.
Personally I would just roughen up the surfaces with 120 grit paper (or thereabouts) and then wipe it down with paint thinner prior to painting with a matte black spray can. (I have used RUSTOLEUM camo paint on a couple of projects and been happy with it.) The spray can paint will adhere reasonably well to the factory paint, better than it would to bare metal.
Using a matte black finish will hide any imperfections and give the tacticool finish to match the G36 stock.
Be sure to post pictures when you complete your project.