What is the difference between wet-on-wet and wet-on-dry painting?

Answer 1

"Wet on wet" or "wet in wet" means that you are applying wet paint (usually oil, artist's colours) to a surface that you have already painted which is still wet. If using oil, then your surface would be oil based, such as linseed oil or turpentine. This technique enables you to move and blend the paint to finish a painting in one sitting.

"Wet on dry" means you are applying wet paint to a dry surface that you have painted earlier and allowed to dry. You can do this to either build up an area, or as in glazing, to achieve certain effects.

Answer 2

There are 4 basic techniques - dry (brush) on dry (paper), dry (brush) on wet (paper), wet (brush) on wet (paper) and wet (brush) on dry (paper).

Each gives a unique look and style.