I've used that method, and didn't like it. Probably because the finest tip I could find at any of the local hobby shops was still a bit too thick, and the scratch marks all ended up with rounded edges to them, if you follow my meaning. You'll need to find a SUPER-DUPER-Fine tipped paint marker, and practice your technique quite a bit before doing it to your helmet.
You can find the markers at a hobby store. I used it on my helmet and it was way easier than other methods i had tried but as propreplicator said it does make it hard not to have rounded edges unless you are extremely careful. I just thought of this, you might try to cut the tip with a razor blade to make it more fine tipped.
I have been using this on a test helmet I have, I found several sizes at an art and crafts store. I have not been really impressed with the result. Like it was stated above it leaves round lines. They do make very fine tipped pens I just didn't get any thinking they would be too fine. In retrospect I should have gotten the smaller one and a middle sized one.
Hey, a method I found that really works well is the testors hobby paint and tooth picks. I just dipped the toothpick in the paint, and ever so lightly ran it across the areas I wanted to and made scratches.
I think that was the thread I started a couple of months ago Bob. I used it in small areas and liked the results. Of course my marker was "micro" found at Michaels craft stores so rounded edges wasn't a problem. I also used the toothpick dipped in silver and it was comparable.
I used a metallic paint pen and it worked wonders. If u can find one in the right color is worth it. Alot easier then the tooth pick method and the paint brush ive tryed both and the paint pen is the best. Its exactly like a brish but its a pen and it has better control. There's a lever at the end u push in to squirt the paint out into the middle of the brush, very easy to use and i think its like 4 or 5 bucks. I just used one brush on all thje scratched and it worked perfectly.