Glazing and scumbling are two different techniques.

Glazing consists of applying thin layers of paint mixed with a glazing medium over a surface, usually to achieve depth and three-dimensionality. One can glaze with light or dark colors. Glazes can also be worked wet-on-wet and made to blend on the canvas. One can glaze over the same surface as many times as one wishes.

Scumbling consists of applying a dry-brush method of skimming the surface of a painting with paint so that some parts are covered and some parts are not. It does not require the use of a glazing medium such as liquin. As a matter of fact it cannot be applied wet-on-wet. One has to wait for a surface to dry completely before applying scumbling. One can scumble with light or dark colors, but scumbling over a surface can be done only once, because over-scumbling creates unnecessary, heavy impasto.