Bill Martin's Guide to Oil Painting
When You Need to Save a Color
Sometimes it is necessary or desirable to save a mixed color to be used again later. In order to do that, you have to stop it from drying. Oil paint when exposed to the air will dry in three days. Once dry it cannot be loosened. Oil paint dries by oxidation. That means it takes oxygen molecules from the air. These methods of preservation are all ways of keeping air away from the paint to prevent it from drying.
The best way to preserve paint is to put it into an empty paint tube. This is good if you need a lot of a particular color. It will last indefinitely this way. Load the paint from the bottom of the tube. Tamp the tube on the palm of your hand to get out air bubbles and seal the end.
Foil packs for smaller amounts of mixed color will preserve a color for years. Seal out the air and label the color.
Plastic wrap can be put over your colors and the palette frozen. This will preserve the colors for about ten days.
Plastic wrap can be put over your palette and will preserve the mixed colors for up to four days. This is best on a glass palette.
Colors can be put under water and will be good for three days. All the water must removed before the paint is re-used.