Hey jroc,
I don't think you overreacted one bit. Pushing her hand away is not a domestic problem. She tried to damage your property and you were protecting what is yours. She is your GF, so nothing bad there.
I would have a serious chat with my GF if she did that. I'm sorry that is uncalled for. You offered to fix her car and you were trying to make her feel better, and she kicks you right in the balls! I would leave the key marks in her car and say sorry I had to spend the time on my car because you decided to run your nails across mine. That is really not cool at all, I know that would never happen with mine so I can't think "well what if". Sorry to hear it jroc, not cool at all.
Take some of her "b-day, anniversary, x-mas" money and buy some new products, and then detail your car again to remove the "nail"marks.

Really not cool, and if anyone says you are overeacting, I think they are wrong. Now if you hit her, or something like that I could see their argument, but you just bitched her out!