…is paint speak for the principle in oil painting of applying paint with a higher oil content over paint with a lower oil content. This principal is important to follow if you want your paintings to “stand the test of time” and not crack because of uneven drying time. It’s not necessary to follow if you are playing around in your sketchbook, or similar, however.

What happens when you don’t follow the fat over lean principal is - as the painting slowly dries over many months, cracks might appear and this creates an unstable surface where moisture can get in, which can eventually cause the paint to flake and fall off.

The more oil in the paint the longer it takes to dry - actually oil doesn’t really dry, it cures, so if very oily paint (where pigment is mixed to transparency with a heavy oil medium) is layered down first and then painted over with a fast drying layer of paint mixed with solvent (solvent dries fast), or lean medium which dries faster than oily mediums but slower than solvent, the top layer dries much faster than what’s under it. When things dry they shrink. So if that fast drying shrinking layer is on top while the under layer is still trying to dry and so it’s slowly moving as it dries, this will cause the top layer to crack because it’s already dry and now that it’s dry it’s also inflexible. Inflexible dried paint can’t stretch with the movement of the drying paint that it’s sticking onto, so it cracks instead of stretching, breaking the seal.

This painting principal doesn’t apply to acrylic paint because acrylic mediums aren’t oily, are flexible when dry, they dry fast and they are binders (meaning they bind the paint together and to the surface. They are a kind of glue. However, thinning acrylic paint with water rather than medium does replace the binding strength with nothing (water evaporates), so very watered down paint that’s dried can easily be wiped off of the surface when brushing on the next layers of paint you put down if you’re not careful. Once the stronger top layers are dried however they bind with what’s underneath and that permanently fixes it.

So keeping these qualities of both oil and acrylic paint in mind, when using them together you should treat acrylic paint as lean, not fat. Also, because acrylic paint dries fast, doesn’t cure over months and is water soluble, it won’t stick to an oil painted surface. Think about it…what happens when you put water onto oil? It won’t sit on top, it won’t stick to it, so it pretty much falls off because it cant bind with the surface, the oil repeals water, and naturally wants to sit on the top.

Exceptions: there are some products that will convert an oil painted surface to accept acrylic paint and these are called universal primers. These are quality products that are designed for industrial and domestic interior and external cladding, and they do work over oil paintings too…and ofcourse you can always deliberately break the rules just to see what happens.

After writing this blog I thought I’d go around my home and see if any of my own oil paintings have started to crack - so far they are looking good and I can’t see any cracks.

and here’s a new oil painting that’s still drying…