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Cook yourself up a big package of bacon, eat a nice breakfast. Drain contents of frying pan into a cup. Go outside with bacon grease and rag. That will annihilate sap for sure. Works like a charm.
I have used WD-40 and also have heard of a secret on using RainX. never tried the RainX but its worth a shot.
Just remember to rewax after all is said and done!
RainX actually works pretty well because of its alcohol content. The alcohol will remove your wax, though, so like you said touching up the area is a good idea.