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