I work on Airplanes and everythign that has been given to us by the Experts that included data from Mt. St. Helens was DO NOT WASH IT OFF instead VACUUM it off. This was going to be a problem with 747 size aircraft as this would take a very long time with a very long extension cord.
Maybe it is different for cars than Airplanes but no way were we supposed to wash it off
that is fine but i could envisage problems with this...
1) in some cases, like on my car, the ash wasn't just easy to vacuum off... it was lightly bonded so wouldn't have come off without addition of a liquid
2) taking a vacuum to a car could cause more damage... i can imagine some might want to use the 'brush' attachment and I don't know what damage that might cause
c) most people only have domestic vacuums which aren't disposable, if you think that you would be heating up the vacuum (most vacuum cleaners get hot) I can see that the very fine particles and glass particles could cause damage to the vacuum itself... most vacuum cleaners cannot even cope with builders dust