Several issues can cause oil paint loss from the surface of a painting. Paintings can be thought of as a series of layers, and artists work to ensure that the layers are compatible with each other, and stay in place for the long term. Archival and museum quality materials are those that are designed to protect an artwork and prevent loss due to the materials aging and changing colour or texture after exposure to the environment. When we plan a conservation and restoration of an oil painting, these issues are of paramount concern.
Several standards related to conservation of an artwork are part of modern conservation and restoration practice. These are careful documentation of current and past conditions and restorations, and the practice of reversibility. Since art historians are able to document a series of restoration and conservation attempts, and the effects of those attempts on an artwork, the standard of practice has been developed that all interventions can be reversed, if we find that over time materials are impacting the work in ways that were not anticipated, or if superior materials and methods are developed.
This principle of reversibility is the basis of current practice in retouching oil paintings that have suffered paint loss. If paint is flaking off of lower layers, such as oil paint from the gesso layer, or paint and gesso from the canvas or other substrate, tiny amounts of water soluble glues can be used to reattach the paint. It is always the goal to preserve as much of the original material as possible.
For areas of paint loss, the painting can be retouched so the enjoyment of the work can continue. This is done within a new layer that can be removed. The original oil paint layer is covered by varnish. This varnish protects the surface; the new retouching, using original pigments such as ochre, ultramarine, or charcoal–pigments that have not changed over time– are placed over the top of the new varnish layer, and then varnish is placed over that new paint to stabilise the image. The process involves separating the original layer from the new, but this is done with such delicacy and a fine touch that the process is usually not visible to the naked eye.