I've done some searching for you on Google, and the best, most consistent
answers I have seen advocate using white household vinegar and/or
muriatic acid to remove dried concrete from painted car surfaces.
(Many people, it seems, spell muriatic as "muratic", so I searched
under both variants.)
The best explanation/instructions I've found is from the following website:
how to remove concrete from paint
URL:
http://www.turbodieselregister.com/forums/archive/topic/50518.html
Quote: "Actually all is not lost. After being a Cement contractor for
20 years, I have run into this on a regualr basis and there are a few
things you can do. First, you didn't say how heavy the splash was, so
I will give you both methods. First if it is light, grab some Vinegar
from the wifes kitchen. Put some in a spray bottle and apply to the
spots or splash. If it is a light "shadow" like splash, this shoud
take it off. However, if you got the real heavy duty chunk stuff stuck
on the paint, you will need the heavy stuff. Find someone with a pool
and borrow some muratic acid (that's right, acid) and put it in your
spray bottle. Get your hose and put your truck in the street or in the
dirt, not your concrete driveway!!! Spray the affected area and after
the concrete starts to bubble for a few moments, wash it off with a
hose. The acid will counter the alkali in the cement and make it wash
off easily. The precautions you need to take are not to get it on
unpainted metal (like chrome or alloy wheels) or on rubber (would
stain it, not eat it). The acid will not harm the paint, and a coat of
wax after your done is all that is needed. Cement truck
owner/operators have used this for years to keep their expensive
investments on the road."
Some other websites that echo this advice:
Subject: "How to remove concrete from car paint"
URL:
http://www.walleyecentral.com/dc/dcboard.php?az=show_topic&forum=3&topic_id=289378&mode=full
Quote: "Assuming the paint under it is good, not a rust spot? Just
pick up some muratic acid ... Dilute it about 1 to 4 with water and
put it on the spots with a foam brush. The concrete should bubble up
and disolve. If it doesn't bubble add more acid. Then flush it
generously with a garden hose. If you spill the acid on the driveway
it will bubble off the concrete there too. Muratic will clean rust
stains on sinks,ect. too but it has nasty fumes so ventilate well."
Auto Detailing: Secrets of the Experts
URL:
http://www.web-cars.com/detail/phorum/read.php?f=1&i=19689&t=19689
Quote: "Jose is correct a diluted solution of muratic acid should
dissolve the concrete without damaging the paint.
You can buy muratic acid from a masonry supply store, ask them what
they recommend as a dilution to use without damaging the paint.
Be sure when you are done to spray the car down with a low pH alkaline
pre-soap solution to neutralize the acid. Rinse it off and then hand
wash the car with a lo pH carwash shampoo.
If you want to polish and wax after that is up to you."
Just please be careful while you're doing this - muriatic acid is
powerful stuff and needs to be handled with care. The following
website goes a little over the edge in describing the dangers of
muriatic acid, but it does have a good section on proper handling and
ventilation:
Muriatic Acid and Cleaning Masonry Surfaces
URL:
http://www.naturalhandyman.com/iip/infxtra/infmur.shtm
