.you can take an airbrush and mix up some very light beige and spray at a distance to front area to simulate sand blast effect from long race.
Take a mix of this same color and do a 50/50 mix with clear with additional thinner and do the windshield to simulate rain or dirt.
Really looks good if you mask off the wiper trail so it looks clean
after you spray windshield.
Also take a 50/50 mix of flat black and thinner, take a Q-tip and dip in paint mix, let a few drops fall offf onto front of car while holding car
verticle, than quickly take air brush and just blow air on the drops so they
will "stream" back to simulate oil from cars that it follows. You can also use this mix with air brush to spray behind wheel wells to simulate raod and rubber debris.
You can also tak some cigarette ashes, fireplace sut, pastel earth
tone colors to simulate dirt scuffs etc without having to gouge up car with
dremil.