With Avengers Endgame released, I wasn't very happy with a lot of the costume makers' choices in how they did the scales for the costume. I tried my best at an option that is easy to print and assemble. Because these are in pieces I think it will be easy to customize for your own costume.
Step 1 - Fill
You're going to need to fill in the print lines, as many of you are used to. The little scales are a pain to fill and sand, but I used the Bondo spot glaze rubbed all over, sanded, sprayed a filler primer, sanded again, then called it done from there.
Step 2 - Attach
The middle chest pieces and the lower chest piece can be glued together, just position them how you want and use CA glue. Keep everything on a flat surface to make it easier. Attaching the middle pieces to the upper chest pieces will require some extra work. I used some scrap styrene I had lying around, but you could use foam cut to shape. Just make sure what you use is thin.
Step 3 - Mold
Once you glue your pieces together, glue them down to a foam core board and make some mold boxes. I did the chest and the shoulders as separate boxes so I could experiment a little with the shoulders more. All in all I used 2 full trial sized bottles of Smooth-On's Oomoo. It was more than one bottle for the chest, then the rest for the shoulders.
Step 3 - Cast
I used Smooth-On's Simpact 60 with blue tint. It cures to a great hard rubber that was flexible and worked with my costume. You can use whatever you have on hand though, at this point
Step 4 - Paint and attach
I airbrushed the pieces to match the rest of my costume's color to something darker. The paint stuck well and it doesn't appear to have any wear. I weather the high points with some silver paint then attached using Locktite's vinyl/fabric adhesive and weights.