While we are on the topic of paint, it appears that someone once painted my
van with a water based paint, a close match (aside from being duller) to
the original paint.  When I wash the van, I get this white residue coming
off.

The dull look is ok, but I'm a bit worried about doing any painting over it
so wonder what would be the best way to remove it - warm water and soap
works slowly.  The shiny area of original paint showing around the gas tank
cap indicates a gasoline soaked rag would be quicker.