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No matter what aggressive method you use its gonna need polished out.There are so many way to get that off seems like you're timid if not comfortable leave it alone.and the paint can withstand a lot of abuse which ever method you pick ,is it tar or cement?
Little late in the thread here,and know you talked to the owner about not going further,but looking at the pic and the usual methods to get rid of the stuff, it would seem if it is really tar? In a way it almost looks like gum. As mentioned maybe a plastic razor. The concern is if it's that adhesive, it would peal the paint off with it when trying to remove. Frustrating i know.
The best product I have found for tackling tar is Turtle Wax Bug and Tar Remover... This is the only turtle wax brand product in my shop, but I aware by it! Spray it on, let it soak for 3-5mins, come back and wipe away! I've tried several products like Trix or TarX and have never had the luck that I have with Turtle Wax Bug & Tar. Almost all local auto parts stores or walmart carry it.
I once got a call from a guy with a brand new silver accord,he ran over a bottle of plumbers dye,it was purple and there was nothing that could be done I tried wet sanding it with 800 grit didn't even lighten up ,took everything possible out of my van and nothing worked ,he had to paint the door he gave 75 dollars for trying though.