Try a pressure washer
, but if the concrete has already dried on the paint, chances are it has stained it.
EDIT
http://www.topoftheline.com/tolae/cement-concrete-remover-auto.html $20.00
For years, our customers have been asking, "How can I remove the cement splattered all over my car?" This is the only product we've found that can do it safely, and effectively.
RoMix Chemical Company developed this molecular cement dissolver as an alternative to using hazardous acids for concrete removal. Back-Set is a one-of-a-kind formula. It uses an active ingredient found in sugar cane syrup, and is completely biodegradable.
The effects of sugar on concrete have been known for many years. In some parts of the world, sugar is commonly used to retard the set of mortar. the December 1957 issue of "Concrete Construction" said, "Sugar affects both fresh and hardened concrete. ... Dry sugar has no effect on hardened concrete, but sugar solutions can be very corrosive."
RoMix turned this vulnerability into an opportunity. "Back-Set" safely removes hardened concrete from a variety of difficult to clean surfaces. It break down the ionic bond in portland cement, releasing the cement from any surface to which it has adhered.
Chemically speaking, Back-Set converts the positive ions in the hardened matrix into negative ions, which dissolves the cement into mud that can be rinsed away. Because it works chemically, rather than mechanically, Back-Set may be used to remove cured concrete, mortar, grout, and stucco from virtually any surface without harm.
Application:
The hardened concrete is saturated with Back-Set, either with a brush or by spraying. After 15-20 minutes, the area is again saturated with the chemical. The concrete typically begins to soften in 20-30 minutes, and can be rinsed off with water.
Specifications:
Biodegradable
Non-hazardous
Non-fuming
Safe for paint, chrome, aluminum, glass, rubber, plastic, brass & stainless
Dissolves cured cement, concrete, mortar, grout, stucco
Cleans aluminum
32 oz. comes with a foaming spray nozzle