Huge improvement, much better! I think you should change your scratches colour on your diffuse to a lot darker grey. Metal is dark, it just has a lot of specular which makes it often appear close to white. So make that darker and keep your spec bright and it should look even better. Well done!
You want to put your mask in the alpha channel of your diffuse. Then you want to use the Alpha Threshold slider with the other alpha options. Translucency is not what you want, that's for something else. Edit: Oh and that shading you're getting, not sure, could be an inverted face/something wrong with the triangulation/non…
Spec maps are used to define materials, your 3 different color paints should probably be the same material and not 3 different ones. Since its all paint it should most likely have very similar values in the spec. (ofcourse there are differnt kinds of paint, some being more glossy and some being more matt and that is up to…