SCUMBLE
scumble
(noun) the application of very thin coat of color over the surface of a picture
Source: WordNet® 3.1
Etymology
Noun
scumble (countable and uncountable, plural scumbles)
An opaque kind of glaze (layer of paint).
Verb
scumble (third-person singular simple present scumbles, present participle scumbling, simple past and past participle scumbled)
To apply an opaque glaze to an area of a painting to make it softer or duller.
Source: Wiktionary
Scum"ble, v. t. [imp. & p. p. Scumbled; p. pr. & vb. n. Scumbling.]
Etym: [Freq. of scum. sq. root 158.] (Fine Arts)
Definition: 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.
Source: Webster’s Unabridged Dictionary 1913 Edition