Here are a couple of details of my painting currently in progress. Those who know how oil glazes work will note the optical greys. Optical greys are thin veils of white which are placed in between layers of glazing color. By scumbling (bringing up whites) in between glazes a painting can be glazed time after time without losing intensity of color. Color can be enriched and refined ad infinitum with this method.
The greys are called optical because there is no actual grey; the painting looks greyish when you put on a veil of transparent white.
Of course, the whites must be completely dry before glazing again, and I use more emulsion than medium to apply them to avoid excessive oil build-up.
When the glazed areas (the flowers) dry, I will once again scumble, let the whites dry, and glaze again.
Glazing this way is time consuming, but I believe the effort is worth the result.