Oiling out is a technique used in oil painting when the oil sinks into the lower layers of a painting, leaving the top layers lacking vitality.
Oil may sink into the lower layers for a number of reasons:
- The lower layers may be too absorbent
- To much solvent has been used
- Not enough solvent has been used
Often sinking occurs in patches, rather than over the whole painting due to varied drying times of the paints used (some colors dry faster than others).
Oiling out is a not a technique which beginners should be worried about. It is more of an advanced technique and is not a necessary part of learning how to paint. It is however a useful technique once one becomes more advanced with oils.
In practice, oiling out would involve dabbing a very thin about of artists' quality oil medium over the areas of the painting which have sunk in, using a fine cloth. This should only be done once the painting is dry to touch, as you do not want any mixing of the colors.