MITRA Forum Question Details

When applying medium or thinned oil to the dry painting, it's best to use only the minimum effective amount necessary to achieve pictorial objectives. One of the main reasons artists oil out paintings is to restore the wet appearance of colors, which makes it easier to duplicate mixtures in a subsequent session. A different painting medium might reduce the amount of color change from wet to dry, and eliminate at least some of the need to oil out. Traditional plant resin varnishes, particularly ones that yield a "candy" gloss, tend to yield good results in this regard but may not prove stable and durable with age. Some alkyd-based mediums can help preserve the wet-mixed appearance of colors, without the drawbacks of materials like damar and mastic.

Other reasons one might oil out a dry picture are to facilitate suave brush movement without "break" in the strokes, and to meld layers seamlessly together. Fully loading the brush can promote smoother movement on a dry surface (to a degree), but most artists who routinely oil out between layers will probably not be satisfied without first lubricating and reducing the absorbency of the dry surface.

When oiling out, the surface should not feel extremely wet- use only enough to allow a well-loaded brush to glide across the surface. It only takes a small amount of oil to facilitate subtle blending and other manipulations of the still-wet paint. 

Da Vinci is known to have broadly manipulated paint with his fingers in creating the "sfumato" effect. ​It seems to me that this approach has something in common with "oiling out".