Since you are new and do not know, what was used in the movie was a product called Rubb and Buff. Also know as RNB on these boards. If you do a search you will see many of us have used it. You can find it in a craft store like Michael's.
What I did with some of my other costumes to create a metal look effect was to dull it down a bit by "weathering". For example, I combined a bit of reddish brown primer to a plastic replica powder rifle, sanded it and scuffed it up a little here and there and it looks so realistic that people would swear it's the real thing. The rifle looks like it 100 lbs. or so but it only weighs about 6 lbs. Also, If you are working with any parts that have "in lays", pits, scratches, lines, ect..I would recommend that those areas to be touched up with the next shade darker to add the illusion of depth and weight. Now it all depends on what costume you are doing like Jango or Boba. This would work better with Boba in my opinion because some of his attire is a bit duller. Jango on the other hand is a bit brighter but with a few scuffs and small dents here and there. I hope my input can help in any way. Good luck!