Two of the most common things we are asked to remove are also two of the most misunderstood. Bird droppings and tree sap are not dirt. They are chemically active, they attack the clear coat while they sit there, and by the time you get around to wiping them off the damage is often already done.
What a bird dropping actually does
Bird droppings are acidic — uric acid, mostly, along with grit and whatever the bird has eaten. Two things then happen at once. The acid attacks the clear coat directly, and the dropping dries and shrinks, pulling the softened clear coat with it as it contracts.
Heat drives both. On a mild overcast day you might have a couple of days before anything permanent happens. On a 35°C day with the car in direct sun, panel temperatures climb well past air temperature and the same dropping can leave a visible mark within hours. Horizontal panels — bonnet, roof, boot lid — get the worst of it because they take the most direct sun.
The test: wash the dropping off and look at the spot in daylight from a low angle. If the paint is smooth and glossy, you got away with it. If there is a dull ring or a faint crater where it sat, that is etching — a shallow dish in the clear coat — and no amount of washing will bring it back.
Tree sap is the slow version
Sap is not acidic in the same way, but it is tenacious. Fresh sap sits on top of the clear coat and comes off easily with a dedicated solvent. Left alone, it hardens, bonds, and over a summer of heat cycling it works its way into the surface. By February, the sap that landed in October is effectively part of the paint.
This is why sap is such a persistent problem in Melbourne's leafier inner-south-east streets. If you park under the plane trees in Armadale or the older streets around Malvern, the car is collecting sap continuously for months, and the drops that landed first are the ones that have had the longest to bite.
What can be fixed, and how
- Fresh, sitting on top. A wash and a dedicated bug-and-sap solvent. Cheap, quick, no lasting harm.
- Bonded but not etched. Chemical decontamination followed by a clay bar to shear off what is embedded in the surface. This is standard in a proper exterior package and it is why decontamination exists as a step at all.
- Etched. Machine polishing. The only way to remove a dish in the clear coat is to level the surrounding clear coat down to the bottom of it, which is skilled work and consumes a little of a finite layer.
The jump from the second to the third is where the cost changes, and it is entirely determined by how long the contamination sat there.
The practical version
- Rinse droppings off the same day, especially in summer. Water and a soft microfibre, no scrubbing — there is grit in there and you will scratch the panel.
- Keep a spray bottle and a couple of microfibres in the boot if you park under trees or near the water. Along the St Kilda foreshore this is genuinely worth the boot space.
- Do not attack dried sap with a fingernail, a blade or a household solvent. You will remove the sap and a ring of clear coat with it.
- A sealant or ceramic coating does not make the car immune, but it does buy time — the contaminant sits on the coating rather than directly on the clear coat, and there is more of a barrier between the acid and the paint.
None of this is exotic. The single biggest factor in whether a dropping costs you nothing or costs you a polishing session is how many hot afternoons it spent on the panel.