Here are four important principles for a stable film in oil painting- from the Winsor & Newton The Oil Colour Book which you can
download from their website here.
• Take care to avoid adding too much solvent to your colour mix.
Excessive solvent spreads the chemical structure too thin, preventing the linkages and the formation of the structural film.
• Always use pure, artists’ grade solvents. Hardware/DIY grade solvents,
or any solvent that hasn’t been refined to the degree achieved with
artists’ grade solvents, often contain impurities that will interfere with
the structural film formation.
• Don’t use old, or oxidized turpentine. To keep turpentine fresh and
usable, always store in full bottles and in the dark. Oxidized turpentine
leaves a gummy residue that can prevent the colour from drying.
• Observe the rules of “fat over lean,” and “thick over thin.” (See the
techniques section on page 75). These techniques ensure that successive layers of colour are increasingly flexible, and are less prone to cracking.