scumbling

/S K AH1 M B AH0 L IH0 NG/
verb
  1. 1

    To apply an opaque glaze to an area of a painting to make it softer or duller.

noun
  1. 1

    An application of scumbling; an opaque glaze.

Source: https://en.wiktionary.org/wiki/scumble, https://en.wiktionary.org/wiki/scumbling