I see yooblem with tree sap I have something similar and have just found that the thing to use is called Clay Bar if you google that you will see what it is and how to use it.
To be honest I've lived with the problem for over a year now and got used to it so hardly notice any more. Like small scratches and dents - it is sometimes better to leave alone.
But it would be nice to cure the problem - will certainly look at Clay Bar
My problem is resin from trees - I parked under some in Portugal last year and didn't do anything about it until sometime later when I found it had solidified and I couldn't shift it with anything. So its still there! ...but I haven't tried Plotter's solution yet.
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