all you really need to do......IS WASH IT PROPERLY)))) i prefer they take it to a real HAND WASH car wash, but i have done a few like this. specially if its only 2 doors or small panels. i use this stuff called PURPLE POWER.
NOW...the trick is how to mix it with water.
right outa the gallon..........this stuff will take off any soft paint. so you dont want that. i would get a bucket with a gallon of water, add 3-6 ounces of PURPLE POWER to it...then wash the door with that.......after it drys..........see if there is any more chalking paint. if there is wash again with a soft brush.......that should get rid of all the dead paint. i dont think its safe enough to wipe down with 91% alcohol either as this will further soften whats left of the paint. tell the client the lettering will probably outlast the paint )))))))))
if its a VAN OR BOX TRUCK....way to much for me to wash...as i tell clients....
i charge as much to wash a vehicle as i do to letter it...........they usually will have it washed PROFESSIONALLY before they bring it to me....at that price))))