I am not sure exactly what you mean when you say scratch, but if you mean you want the "metal" to show through the white then i can help you with that.
Step 1: Take some mustard from the kitchan, or toothpaste from bathroom.
Step 2: With whatever kind of paste you chose ( i am gonna referter to mustarrd throught this awsner) bring your armor inside, over the kitchen sink.
Step 3: You should have already painted your armor the "metal" color, but not your armor color
Step 4: Take your Mustard, and open the cap.
Step 5: Start put lines of mustard onto your armor of where you want the silver to show through the color of your armor (usually around edges, and high points)
Step 6: Paint your armor however you want to after you put on the mustard (leave the mustard on until you are done painting!)
Step 7: after everything is dry, take a cloth, or paper towl or whatever, and start to wipe off the mustard. it will take a few passes because the mustard will squirt out and other stuff...
Step 8: Make sure you have gotten all the mustard off, and then where the Mustard just was, the silver paint will be showing through!
Step 9 (Not needed): Mix black, grey, and white water based paint (like the apple barrel brand) together in a bowl, add some water, and then take a brush, and just slather it on thickly and quickly. then, quickly wipe it all off with a paper towl, it should leave dark streaks and shadowing..
Step 10: Seal your armor with clear coat!
Hope this helped!