Had the same on my car.
You will need an aggressive clay to remove the grinding dust http://www.polishedbliss.co.uk/acata...cts-clean.html

Will take a while to fully remove and you will need to polish afterwards but you will be able to get it back to its former glory

May also be worth reading the advise on decontaminating paint http://www.polishedbliss.co.uk/acata...ing-paint.html

Hope this helps mate