Fallout from the Icelandic volcano can damage the car paint. There is no chemical hazard at all, but the ash may produce heavy scratches if no precautions are taken when washing your car.
If you have an unnatural surface in the form of a thick mass of the car, then don't use a sponge to clean the car!
Clean the car with a high pressure washer instead - and be careful when doing so - because the pressure washer may push the ash deeper into the paint and make scratches.
Make sure the ash is cleaned away before using a sponge to clean the car more throughly.