The cracks that form in decoupage painted surfaces are often intentionally created to achieve an antique or distressed look. This effect is usually achieved by applying a layer of special crackle medium, which is a type of glue that dries with a cracked appearance, over the base coat of paint. When the topcoat of paint is applied over the crackle medium, it shrinks and cracks as it dries, revealing the base coat underneath.
The cracks are formed because the top layer of paint dries faster than the bottom layer due to the difference in thickness and moisture content. This causes the top layer to shrink and pull apart, resulting in the appearance of cracks.
After the paint has dried and the desired level of cracking is achieved, a layer of varnish is applied to protect the surface and make it durable. The varnish also helps to enhance the crackled effect by filling in the gaps and giving the surface a smooth finish.
Overall, the natural paint cracks are formed as a result of the deliberate application of a crackle medium and the drying process of the paint and varnish.
