Why water will not touch it
Sap is a mix of terpenes and resin acids that are not water soluble. Rinsing, however thoroughly, leaves it exactly as it was. Once sun has cured it for a few weeks it also cross-links and hardens, so an old sap spot is meaningfully harder to remove than a fresh one, which is why timing matters as much as the removal method itself.
Does the tree species actually matter?
It does, and not always in the way people assume. What most homeowners here call sap dripping from a shade tree onto a car or window is frequently something else entirely: aphid honeydew, a sugary insect excretion that maples, lindens and other broadleaf shade trees host heavily through summer. Honeydew is sticky but water soluble, and it wipes off far more easily than true sap. Real resinous sap is more associated with conifers, and with the sticky bud coating on black cottonwood, one of the more common trees along this region's waterways and greenbelts, which carries a fragrant, resinous varnish on its buds that behaves much like conifer pitch once it lands and cures on glass. Knowing which one you're dealing with changes both the urgency and the method.
Why picking at it damages glass, and why glass is more forgiving than paint or vinyl
Hardened sap feels like it should chip off, and it partly does, taking a bit of whatever it is stuck to along with it. Glass is chemically simple and has no finish layer to lose, which makes it the most forgiving surface here: a solvent strong enough to fully soften cured sap, such as isopropyl alcohol or mineral spirits given real dwell time, generally will not harm bare glass. That same solvent can haze a vinyl sill or strip a painted wood frame, which is why the identical bead of sap sitting across a windowpane and its frame needs two different products, not one all-purpose approach. Dragging a blade or fingernail across hardened sap on glass is still a straightforward way to scratch it, and on tempered glass a blade can leave permanent marking, so softening always comes before any mechanical lifting.
What actually works
Sap is softened with a solvent appropriate to the surface, given real dwell time, and then lifted rather than scraped. On glass that generally means a stronger solvent and a confident approach; on a painted frame or vinyl sill it means testing in a hidden spot first and accepting that it will take longer, because you cannot use the aggressive product that would finish the glass in seconds.
Prevention is mostly positional
Where a branch overhangs a window or a parking spot, sap will keep arriving regardless of how well the last batch was removed. Pruning back is the durable answer. Where that is not possible, dealing with sap while it is fresh, before UV has had a few weeks to cure and cross-link it, is far easier than removing a hardened bead months later.
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