MITRA Forum Question Details

​My first advice would be to make sure it's absolutely necessary to overpaint an earlier work rather than starting fresh. Any time an oil painting is overpainted for revisions, there's a good chance that pentimenti (visible ghost images of the original work) will result. That's not necessarily the worst thing in the world- pentimenti can become a fascinating record of process- but if earlier work persists through a major design element, it can be distracting. 

The relative freshness of the piece you're describing likely lends itself well to resuming work, in terms of accepting new paint. If the paint is rather thin and will not require scraping or sanding to flatten paint texture, it would be a good idea to wipe the surface of the passage to be altered with a cloth moistened with odorless mineral spirits (OMS), then allow it to dry. This will remove the top glaze of oil and render the surface receptive to new color. 

​The darker areas in your painting could indeed come through as pentimenti, when the oil paint becomes more transparent with age, even if only faintly. 

Since you already have a paint layer down, you should not use thinned-out washes when continuing to paint as you will go against the general advice of 'fat over lean'. If there are areas of raised brushstrokes, or very smooth and glossy areas, they will need to be sanded to help provide some tooth for subsequent layers to grab onto. The ideal would be if the painting has an overall matte to semi-matte appearance. Oiling out the areas that you will paint into with just a small amount of linseed, or stand oil with some odorless mineral spirits, can be helpful in terms of paint handling and color matching. Just make sure to thoroughly wipe off any excess before painting, leaving just the thinnest and barest of layers to work into.

​Definitely do not apply acrylic dispersion painting ground (gesso) over top of existing oil paint. 

​Cremnitz white/ lead white produces a more transparent paint in linseed oil, compared to titanium. Therefore, a base of titanium white would probably be the better choice to prevent pentimenti. The transparency has to do with the refractive index of binder and pigment. The closer the refractive indices of pigment and binder, the more transparent the paint. Linseed oil has a refractive index of about 1.5, lead white pigment of 1.9/2, and titanium white pigment of 2.7. As the linseed oil films age, its refractive index rises and thus paint becomes more transparent.   

​Please do a search here on MITRA about pentimenti and increased transparency. Certainly oil does raise slightly n refractive index over time. However, many now believe that most of the increased transparency now seen in paintings of the past is a result of the formation of metallic soaps which lower the opacity of the paint film. Lead and zinc white pigments readily make lower refractive index soaps in fatty acid environments. It is not known whether titanium dioxide will create the same reactions over time. 

Brian Baade