I have this
problem as i have a 90 year old oak tree overhanging my driveway..the best thing for this is HOT water, which will desolve the tree sap (it is only xylem and phloem excreeted from aphids as they feed on the tree sap)- so its sugar water really- and if the
problem is really bad then use a clay bar, i did this a week ago and my paintwork is like glass again..wicked stuff, thenjust make sure you lay down a coulple of good coats of wax to seal it, and the tree sap should just wash off next time...
Matt