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Bird Droppings and Paint Damage (Etching)

a photograph of bird droppings causing etching on painted car surface

Ah, Springtime. The air is warming; flowers are beginning to bloom. You venture into the driveway to give your car a well deserved wash after enduring yet another winter of corrosive road chemicals. After completing your extensive auto detail, you step back with your hands on your hips to admire your handiwork, and you notice that group of birds has decided to show their admiration with an… er… offering.

Bird droppings. Bird deposits. Bird excrement. Bird poop. Call it what you will, it’s not a pretty subject – and, it’s doing real damage to your car. Here, we’ll discuss paint etching as a result of bird droppings; why it happens, and what to do about it.

Bird droppings contain Uric Acid – a chemical that is corrosive enough to quickly eat through a coating of wax or paint sealant and begin to etch paint.

The Biology: Probably more than you ever wanted to know about bird poop.

Unlike mammals, birds have no bladder – they don’t store liquid waste separately from solid waste. All of their waste is mixed together in an organ called the cloaca. Solid waste from the intestines is mixed with concentrated uric acid from the kidneys and everything is eliminated together. Thus the watery bombs we’ve come to know. Uric acid crystals add the white coloring to the waste matter. The uric acid levels in bird droppings is relatively high; reaching pH levels of somewhere between 3 to 4.5 – quite acidic.

The Chemistry: Why acids tend to break down surfaces they come in contact with.

pH is a measure of the levels of Hydrogen (H+) and Hydroxide (OH) ions in a solution. When the Hydrogen and Hydroxide are present in equal levels, neutral H2O is formed. Neutral things like water or milk have pH levels of 7; the H+ ions and OH ions are in equal number. When Hydrogen ions outnumber Hydroxide ions acids are formed. Acids like lemon juice or uric acid have pH levels ranging between 0 and 7. Nature likes things to be balanced. A solution like uric acid, with extra Hydrogen ions will try it’s best to get back to a nice calming neutral by grabbing Hydroxide ions wherever it can find them. So in terms of automotive finishes these extra H+ ions found in the uric acid of bird droppings will react with the hydrocarbons of the finish – slowly breaking down the clear coat finish.

The Physics: That sunlight is doing more than reflecting beautifully off your freshly detailed car.

There is more at play here than simply biology and chemistry. There is also a good deal of thermodynamics at play. Wait, what!? Yes, all of the sciences have come to play. If we remember way back to high school physics, we are reminded that heat causes expansion and cold causes contraction. As heat is added to molecules they tend to move around and bounce off of each other at a faster rate. As molecules warm up and cool down, the physical space between them increases or decreases. Metals are good examples of this thermodynamic property. Ever hear the copper pipes banging in old houses with hot water heating systems? The banging is a result of the copper piping heating up, expanding and banging against the framework holding the pipes in place. The same thermodynamic principal effects the metal of our car bodies as well. As the sun warms the car metal during the day the metal expands slightly. Paint and clear coat also expand in the warmth of the sun. In fact, the clear coat becomes porous and allows some of that acidic uric acid to penetrate. At night, as everything cools down, things start to contract and the clear coat starts to harden again. Only now it hardens around the newly incorporated uric acid. Over time, this constant expansion and contraction allows the uric acid to slowly etch into the clear coat and into the base paint coat.

As a result, on a macroscopic level, the painted surface appears dulled or etched where the bird droppings were. This is a result of reflected light being interrupted by the now irregular and deformed surface.

So, there you have it. The reason why bird droppings cause paint etching on cars.

Oh, wait! What do you do about it now that it’s happened? Ah, good question. Well, as our fathers would say, “prevention is the best remedy”. In this case, they’d be right. Remember, we’re working against the very Laws of Nature here: Biology, Chemistry, and Physics! The best option is to prevent the etching from happening in the first place. Covering or garaging a car will obviously keep the bird droppings from contacting the car’s painted surfaces.

But again, we’re up against the laws of Nature here. Specifically, the Laws which state, “Thy freshly washed car shall become the prime target for local birds” and “That which goes up, must come down”. Acidic etching can happen in a matter of minutes on a hot car surface. Keep a small bottle of spray detailer and a clean, soft microfiber polishing cloth with you for just such emergencies. Acidic reactions begin to happen almost immediately, so time is of the essence. This is also a good advertisement for waxes and paint sealants. They will offer some short lived protection; give you a little bit longer to address the situation before the damage occurs. The quicker you can remove the bird droppings, the better.

And what if the etching has already happened?

There are basically two different types of etching that can occur on automotive surfaces: Topical Stain Etching, and Wrinkle Etching. Topical Stains are shallow dull spots resembling hard water stains on the clear coat surface. However these stains will be in the typical bird dropping shape (picture something splattering against the surface). In this case the etching is only taking place on the surface and can usually be corrected with compounding, buffing, or in difficult situations, wet sanding. Again, it is important to remember that time is of the essence. The quicker an acid etch is addressed, the more likely it can be corrected without too much work.

Wrinkle Etching, or Fracture Etching is more problematic. This type of etching is the result of the thermodynamic heating and cooling combined with acidic etching. Here, the paint and clear coat fracture as they expand and contract, causing deep fissures in the clear coat surface. At this point, the damage has usually penetrated the clear coat surface and is beginning to damage the paint layers. Etching that has reached this stage usually requires repainting.

In either case, it’s time for a professional automotive detailer or paint correction specialist to address the situation.

And there you have it. Probably more than you ever wanted to know about bird poop and why it is important to clean it off of your car. We’re nothing if not thorough here at AQS!

  1. Really interesting read, thanks!

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21670 Red Rum Drive, Suite 149
Ashburn, VA 20147
Phone: 571-291-9401
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