Painting into a Dry Painting
When a painting dries, the color tends to go flat. The lights and darks get duller and flatter. So when you want to paint back into a dry painting you have to do something to bring the colors back to their original state and value. There are several options you can use. Retouch Varnish can be used and comes in spray or brush-on form. The smell of the varnish is pretty strong and over-powering.
Oil Medium is also an option. Brush it on and then wipe it down so the medium isn’t too thick. Oil medium dries real slow which can be a problem. Another option is Liquin which is an alkyd medium. It dries quickly (over-night) and brings the dry color back to its original state. Liquin does have an order, not as strong as retouch varnish and it can’t be removed, it’s permanent. As long as it’s not applied too thick, it does a good job of restoring dry color.