Just a guess, but have you tried that Bug, Tar, & Tree Sap remover? It might work, and shouldn't hurt the car. When I've gotten door dings on my cars before, I would remove the paint from the ding by using gasoline on a rag without damaging my own paint (Gas runs down the side when filling the tank sometimes and doesn't hurt the paint). Whatever you do, clean the area first and don't rub very hard. If you get it off, put a coat of wax on the area afterward.

Hope this helps.

Dale