Noticed a very fine white powder on my plain black car. Seemed to have etched itself into the paint as it wouldn't wash off. Tried some Farecla which did shift it but scratched the paint in the process.
Trying a clay bar was suggested - and I'm delighted to say it worked beautifully. Not hard work either - removed all the surface crap and left a smooth scratch free surface which when waxed looked like new.