Are you strictly going for a diffuse only look? If you want it to look more "metal" you can use PBR. It works with hand painted. In general, metal is very shiny, unless it's rusted or has grunge added to it. You could fake those reflections in your diffuse but they might look weird in a game (because that information gets…
Metal can be really tricky to paint with just diffuse. What makes metal look right is reflection, otherwise it will just looks like stone. Try to paint in some generic reflection. Add gradients and a strong highlight.