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.