Bird Droppings and Tree Sap on Car Paint

Why droppings etch

Bird droppings are acidic — uric acid chiefly — and clear coat is not fully resistant to acid. On a cool, shaded panel the reaction is slow. On a warm panel in sun it is fast: the clear coat softens slightly with heat, the acid works into it, and within a few hours there is a dull outline of the dropping etched into the surface.

The heat is the multiplier. A dropping on a car parked in shade in April may do nothing for a day. The same dropping on a black bonnet in July sun can mark the paint in an afternoon. This is why a car parked under trees on a hot weekend so often comes back with permanent spots — not because there were more droppings, but because each one had heat and time.

Once etched, the mark is a change in the surface of the clear coat. Washing does not touch it. It needs polishing to level.

Why sap bonds

Tree sap is resin. It lands as a liquid, and as the solvents in it evaporate it hardens into something close to a varnish, chemically bonded to whatever it dried on. Wisconsin's maples and pines produce a lot of it in dry spells and after pruning.

Fresh sap wipes off with a damp cloth. Sap that has dried for a week does not wipe off with anything — it has to be dissolved. And sap left for a summer hardens to the point where removing it takes the surrounding clear coat's gloss with it, because the bond to the paint is now stronger than the paint's bond to itself.

Sap does not usually etch the way droppings do, but it holds dirt against the surface and it can leave a dull ghost where it sat if it was on long enough.

Safe removal at home

Droppings: as soon as you see it. Soak a microfibre in water, lay it over the dropping for a minute or two to soften it, then lift straight up rather than wiping across — droppings contain grit that will scratch if dragged. Rinse the area. If it has already dried hard, more soaking, more patience, no scrubbing. A dedicated bug-and-dropping remover helps.

Sap: fresh, a damp cloth. Dried, a dedicated tar-and-sap remover or isopropyl alcohol on a microfibre, dabbed on and left to work for a minute before lifting. Do not use anything abrasive and do not use a fingernail. Rinse and, if the car has wax or sealant, reapply to that spot — the solvent will have removed it.

What not to do in either case: let it sit because you will wash the car at the weekend. The weekend is too late for a dropping in July.

Once the damage is done

A dropping etch is a depression in the clear coat and the only fix is correction — polishing the surrounding surface down to the base of the mark. Light etching comes out with a single-stage correction; deep etching, where the acid got well into the clear, sometimes cannot be fully levelled without removing more clear coat than is safe.

Prevention is the honest answer for a car that parks under trees. A ceramic coating does not stop droppings landing, but it puts them on the coating rather than the clear coat, and the hours before you get to them do far less damage. Our Cedarburg page is about exactly this problem.