Oiling Out: to brighten dull areas and prevent 'sinking'
As ever, prevention is better than cure, and the best way to avoid ‘sinking’ on oil paintings is to apply artists’ primer such as Oil Painting Primer to the surface before you start.
However, as you build up oil colour in layers, the different drying rates can also result in a paint film with varying degrees of absorbency, which show up as dull spots rather than a complete dull area. The trick is to treat these dull areas as soon as they are dry. Apply Artists’ Painting Medium sparingly to a clean cloth and rub gently into any sunken areas. Wipe off any residue and leave to dry for a day or two.
If you can still see smaller, dull areas then repeat the process until the painting has regained an even sheen. Avoid using varnishes to refresh a dead painting. For a faster drying oiling out medium, use Thickened Linseed Oil diluted with 50-percent white spirit (mineral spirits).