Fat over lean violation leading to cracks
“My oil painting developed spiderweb cracks as the upper layers dried.”
What is happening
Fine cracks, alligatoring, or crawling wrinkles appear across upper paint layers. The top coat separates and splits open, revealing underlying colours.
This happens when the 'fat over lean' rule is broken. Applying a fast-drying, low-oil (lean) layer over a slow-drying, oil-rich (fat) layer causes the top skin to dry and contract while the flexible lower layer is still shrinking and moving underneath.
The 30-second test
Review your medium mixtures: did you use pure solvent over paint containing heavy oil medium or straight tube black?
How to fix it
- Let the painting dry completely for several weeks to halt further movement.
- Do not try to force paint into active cracks while the underlayer is still wet.
- Once fully hard, gently scrape away raised or detached crack edges with a painting knife.
- Infill the valleys with stable oil paint prepared with balanced medium, and strictly adhere to fat-over-lean on future layers.
Can it be saved? Partly
Minor cracks can be filled once the whole structure has dried over months, but deep structural cracking cannot be undone cleanly.
Drill: Medium ratio layering chart
About 15 minutes. You need:
- Oil paper or canvas scrap
- Linseed oil
- Mineral spirits
- Earth paint
Mix three swatches: lean (lots of solvent), balanced (50/50), and fat (lots of oil). Note the drying times over several days to understand film elasticity.
Background reading
General teaching sites on this subject. They are a reading list, not page-level citations for this advice.
The advice is a generated synthesis of common teaching practice and has not been checked line by line against any source; treat times and difficulty as estimates.