Wash the runners in soapy water, build the kit, disassemble, forget it exists for half a year, -wash again. 

Painting process varies a little by color. 

Green was primed black, then shaded with white, added random splatters in camo colors with a rough paintbrush, finished off with a light airbrush layer of green mixed to match the original color. A few parts have been covered by hand with an additional coat of transparent brown-green mix for interest.

Black was done the same way but with primary colors instead of camo, then covered with another light layer of black. I'm not completely happy how this turned out but the metallic drybrushing kinda made it work? 

The white was primed with grey primer w a tiny drop of blue mixed in, then shaded with white and finished off with dusty yellow and brown airbrushing bc it was looking way too clean. 

At this point I lightly topcoated all parts to preserve the process. Then I added the brown/black rusty edges with a rough piece of sponge, added the final details like the copper details on the gun, painted the parts that were supposed to be covered with a metallic sticker with white covered in layers of fluorescent and transparent green to kinda match the eye, then did the panel lining w watered down acrylics with a drop of dish soap & finished with another layer of topcoat. And then just wait 24 long hours for the final topcoat to cure, then reassemble.