If the unwanted paint chips are not near your blacklines 
a q-tip with a little bit of alchohol will remove the paint.  
But be forwarned it will also remove the lines on the 
surface of the cel, so be very careful!   This cel had a 
very stuck top layer of flying rocks, the largest was right 
over her face.  I lost some black but it seemed to work 
well.  I tried scraping but the force required marred the 
surface.   Hope this helps.