1) You can turn in a comp claim if you have that insurance coverage. Your insurance company will ask/require you to report this to the PD> (GOOD IDEA IN ANY CASE) < ** If you use your insurance, they will then subrogate your SD, and recover the money. (including your deductable.) ** This is why they need the PD report, to have reason to subrogate. (Sue him.)
2) You can sue your SD in small claims court. You should report this to the PD NOW.
3) You can continue to live with this, and do nothing, and next time, it might be your head being smacked by the hammer. (or your Mom's, or your other family members.....) Get my point? This guy needs a reality check, and some counseling. (I hear they offer that free in prision...)
I feel for you because this is not an easy situation. He is not your Dad in the real sense, and appears not to be making any effort to play the part. I'm more worried about your Mom and other family members than I am your fender and trim. That stuff is easy to repair, but broken people are not. If he's hitting your car, he's hitting your Mom and brothers/sisters. Anger does not just surface at one person. And this guy is angry at something. He's not angry about this putty knife, he's angry at life, and your part of his life. (Lucky you eh?)
I know it's not easy, but you need to talk to a police officer RIGHT NOW. Go down to the local PD, and ask to talk to someone who specializes in family conflict. Calmly explain what went on, and if you want, have your witness also talk to the PD. They will likely record your statement, so don't make anything up, just tell it like it is. (Not easy as emotions tend to sway your thoughts.) The PD can then decide on what course of action to take. It may be tough, but if it saves your life, and/or the life of your family members, it's worth it in the long run.
That being said, IF I had a SD, and he threw a hammer at me or my car, I'm not sure what I'd do. I would likely talk to the PD in any event. Find out what your options are according to them. Never hurts to have a friend in the PD, especially as I don't think this is going to go away easy. (Even with Siman's idea of a guilt trip.)
Either way, good luck with this difficult situation.