When the copper paint dried, I peeled off the mask and then shot a clear coat of paint over the whole cartridge. The clear paint helps to seal in all of the tiny pin holes, small imperfections, and tiny rough surfaces that is almost invisible to the eye. This clear coat sealing will help with the next step.
This step I used simple water colors to help bring out the details. In the plastic modeling world, this is called a "wash." Oftentimes, they will use various artist's oils heavily diluted and thinned out as well as various dark-colored paints. The paints themselves can ber acrylics, enamels, and even water colors. Notice, I used the words "dark colored" and not black. Black is too extreme. These dark colors can range from browns to orange, and even greens, blues, and yellows depending upon where they're applied. When heavily thinned out for washes, either paints or oils can flow easily through all the small cracks, crevices, pin holes and more, highlighting some areas and bringing out the shades and details in other areas. In my case, I wanted to do the latter. That is, to bring out shades and details that were hidden from the copper spray paint.
I simply diluted the water colors with lots of water to where the colors themselves were transparent. I then allowed the dark colors to flow over all over the cartridge and settle. You can use water colors over dried acrylics. Acrylics are water-based paints.
Next, I used a cotton swab diluted in water, to wipe off the excess colors off of the areas I wanted to highlight, and leaving the crevices or corrugations dark to help bring out the details. I repeated this several times slowly building up the details and highlighting areas with each step until I was satisfied with the end results.
I added the ring that I trial-fitted earlier. I purposedly left it unpainted as I wanted to bring out the metal contrast against the background. Next, I cut a thin strip of glossy paper and painted it a light tan color to simulate the leather. I then bent the ends of the thin strip to attach to the ring to simulate the leather strap. Voila' there you have it. A finished drum cartridge to be fitted over a nice machine gun.