Every time you take your car out of the garage it's exposed to potential paint damage hazards. Some of the most common issues are acid rain, insects, battery acid and road stones. There is another hazard that's particularly messy and harmful, and that's bird poop.
Bird poop is essentially an acid. It can have a pH level between 3.5 and 4.5. That means it will burn delicate surfaces without it's removed relatively quickly. On car paint, in particular clear coat finishes, bird droppings burn the surface, causing a permanent etch mark.
It's not at all uncommon for a large dropping to cause paint damage that's 2 mils deep, or more. To understand what that means to your car's clear coat, good laser printer paper is about 2 mils thick. More importantly, a quality clear coat finish is typically 4 to 6 mils. That means a heavy-duty bird bomb will not leave a lot of clear coat in the area that needs to be repaired. The good news is that paint etching from hard water falls and bird droppings is an easy repair you can make at home. Most repairs like this take less than 5 minutes.
Fixing Bird Poop Paint Damage
The only way to repair the paint damage caused by a bird is to polish the affected area with a fine cut rubbing compound. The basic procedure is to use the compound to cut away the damage and re-polish the surface so it looks new again. This is exactly the step taken by professionals and the factory when they remove painting defects. Your only concern is over-polishing. Be careful that you do not over do it and polish all the way through the top coat.
The best car polish to use is one that's formulated for removing paint scratches or restoring oxidized paint. The product I highly recommend is Meguiar's Ultimate Compound. It's available in most auto parts stores, and it works great by hand or with an electric car polisher.
If the damage covers an area larger than about 1 square inch, a polishing compound alone might not do the trick. For larger areas of damage you can make the repair faster and easier by first removing the damaged paint with 2000 or 3000 grit wet sanding paper, then re-polish the area with the compound. This may sound severe, but it's not. It's a safe procedure used by paint and body shop professionals.
Preventing Bird Damage
Although it's difficult to avoid being splattered with bird waste, it's not at all difficult to prevent the damage. The sooner you remove the mess, the less likely it will be that your paint will be damaged. Conversely, the longer the mess remains on your vehicle's paint the more damage it will create.
If you live in an area where you are preaching to pooped on, I highly recommend keeping a small bottle of car detailing spray and a microfiber cloth or two in your vehicle. It will come in handy for all sorts of spills and emergency cleanups. The Meguiar's Quik Detailer product is very inexpensive and easy to find. When you need to clean up a bird mess, simply give the bird droppings a few shots of the spray, allow it to soak for a moment or two, then wipe it away.