The best approach to solvent free painting is to execute the painting in one application, thinning the color with a small quantity of Walnut Oil or Walnut Alkyd Medium. For the greatest permanence, the colors should be applied thinly. When multiple layers of color are required, the technique is quite simple if a few rules are remembered.
Paint Thinly – Heavy applications of color are too massive to age well and are generally liable to wrinkle or cause cracking. It is better to apply color in a fashion that assures that the canvas texture is not lost.
Fat over Lean – or flexible over inflexible. The first coat of color should have little or no medium. Each successive layer should have slightly more oil or medium added to it than the underlying coat.
Thick over Thin – Thicker layers of color can be applied over thinner layers of color. Often when thin layers of color are applied over thicker layers cracking can occur. (Zinc White acts as though it has a high oil content and must be used carefully in underlying applications. It is not generally recommended for use in underpainting unless applied very thinly on a porous ground and allowed to dry thoroughly.)
Note: Glazes applied with appreciable quantities of medium in exceptionally thin layers are an exception to this rule.
Slow over Fast – Slow drying colors such as Titanium White, Cadmium Red, etc. should be applied over faster drying colors such as Burnt Umber to avoid cracking. In addition, sufficient time should be allowed for the underlying layer to dry thoroughly.
Use a sufficiently porous ground with “tooth”…oil colors adhere by mechanical adhesion. This requires a ground coat with some surface irregularity that the oil can sink into. Gesso should be applied thinly to preserve the texture of the surface for adhesion of the color.
Use the same medium throughout the painting…this will help to avoid difficulties in the painting structure that can lead to cracking due to uneven drying rates.