Causes:         
    In industrial regions, tiny particles of dust or ashes which are
    propelled by the factory chimneys may land on the vehicle and
    stick to the paint. The hardened paint becomes rough to the
    touch and the particles may very well become visible.  Even
    though this problem is not related to the paint or paint
    application, it still needs to be repaired.

Remedy:
    – A light contamination when the particles have not penetrated the paint film may be
      removed with light sanding and polishing.
    – A more serious contamination may only be removed with oxalic acid.

Note:
    Oxalic acid is a poison.  Avoid all contact with skin and eyes. Wear protective goggles
    and gloves.