The Collaborative International Dictionary of English v.0.48:

Scumble \Scum"ble\, v. t. [imp. & p. p. Scumbled; p. pr. & vb.
   n. Scumbling.] [Freq. of scum. [root] 158.] (Fine Arts)
   To cover lighty, as a painting, or a drawing, with a thin
   wash of opaque color, or with color-crayon dust rubbed on
   with the stump, or to make any similar additions to the work,
   so as to produce a softened effect.
   [1913 Webster]

WordNet (r) 3.0 (2006):

scumble
    n 1: the application of very thin coat of color over the surface
         of a picture