“Sinking in” and how to fix it

Sinking in is a phenomenon that really all oil paintings experience during their drying phase. There are many explanations for why this happens, most of them are not supported by contemporary scholarship. In this post, I’ll try to explain my understanding of this phenomenon and what I do to fix it (and why).

First, let’s take a look at a painting that has fully dried and examine the “sinkin in”, which exhibits as a dulling of the colors and a loss of “shine” on the surface of the painting.

Dry painting that has dulled, often called “sinking in”.

The effects of adding varnish to the dry painting, restoring the color and “shine”.

You can see in the top image that the black isn’t very black and even the background looks a little dull. When adding a layer of varnish, the blacks are restored and the background regains its shine.

So what’s going on? Many online articles and artists explain that “sinking in” is actually the oil from the paint literally sinking into the canvas, as if the layer beneath, be it oil primer, acrylic primer (AKA modern “gesso”), classic gesso, or whatever, is actually drawing out the oil from the paint, creating the “matte” looking layer above.

However, this “sinking in” effect can be seen even on supports that are 100% non-absorbent, such as shellac sized panels, and even glass. So, we know that’s not the cause of the sinking in. (Note that it may be possible that oil can be drawn into a lower layer in a painting, I really don’t know for sure if that’s not happening on a given support, but I know for sure it’s not happening on shellac, and we still get the sinking in there.)

Another explanation for sinking in is something called “under bound” paint. This theory suggests that certain pigments are very absorbent and paints made from them need to be made with a sufficient amount of oil in the suspension process (mulling or rolling or whatever). If not enough oil is used to create the paste we call paint, then the pigment will absorb too much of the oil and the result will be this matte look we call sinking in. Again, the data doesn’t support this. Firstly, because this phenomenon happens to any pigment given sufficient time and environment. Secondly, the amount of time paint stays in the tube is generally far longer than it takes for paint to dry and “sink in” after it’s smeared on a surface. If the pigment were to “absorb” the oil, it’s reasonable to expect that to happen in the tube.

One common remedy often suggested for both of these causes is “oiling out”, which is just adding a layer of oil to the dried paint surface. This has the immediate effect of restoring the original luster and color richness of the painting, much like the varnish did in the photo above. Unfortunately, since the root cause in both cases is incorrect, this is not a permanent solution. Once that layer of oil dries, the “sinkin in” returns, where another round of “oiling out” is often suggested. This can go through many rounds before final varnishing.

So if the above explanations don’t stand up to the data, what is happening? There are clues in the above explanations. Once the paint is out of the tube and on a surface, it’s exposed to air, containing oxygen. This is where the autoxidation process begins. Certain pigments are faster to oxidate than others. Notably, earth tones are made from clay containing iron oxide, which speeds oxidation, whereas titanium does not, for example. I believe the most credible theory is that autoxidation is the actual cause of the “sinking in”. During the drying process for oil paints, the surface structure of the paint actually changes at a molecular level. This causes the light that reflects off of the paint to have non-uniform angles of reflection across the surface, causing the matte finish. This is how matte varnishes work, by the way. A matte varnish typically has wax in it specifically to reflect light in a non-uniform (and slightly occluded) way.

Since the cause is the natural shifting of the surface structure of the paint as it dries, adding oil only repeats the process, though it may eventually even out with pure oil and no pigment. But each time you “oil out”, you are adding a layer to your painting that becomes part of the final paint film. And you still need to wait for this to dry before varnishing, so you’re really just delaying that final step. Oiling out can be useful tactically, if you need to restore the color and shine to a painting for a show, for example, but it’s not really a durable fix for the problem.

The easiest way to fix this issue in a durable way is to wait until the painting can be varnished, and then let the varnish even out the surface of the paint as in the above photo. There are different kinds of varnish. Some are thicker than others. Some are synthetic and some are “natural”, but all of them do essentially the same job. I personally favor a thicker varnish as I believe it gives me better results. Thin varnishes (like Gamvar) will restore the colors and shine but since they are so thin, they don’t fully even out the surface of the paint and this can lead to having more glare across the surface. This is something that not all artists need to worry about. If you paint using thick paint, you are probably not worried about “flattening” the surface to avoid glare. You just want to restore the chroma and shine to your paint surface. In that case, Gamvar is a good choice for final varnish.

The reason we usually wait so long to varnish is to avoid the varnish becoming a permanent part of the paint film. If we varnish too soon, we won’t be able to remove the varnish at a later date (e.g. for cleaning). Gamvar avoids this as it can be removed easily with a gentle application of Gamsol solvent, so it can be applied as soon as the painting is “touch dry”, which is a bonus for people who choose Gamvar. There may be other varnishes, or recipes to make your own, that have similar properties. But Gamvar is the only commercial varnish I’m aware of that can be applied so early in the drying phase. (Note: retouch varnish can be used as soon as paint is touch dry also, but it’s not considered a “final varnish” like Gamvar.)

In conclusion, “sinking in” happens to oil paintings as they dry. Oiling out can temporarily address this if you need a quick fix, and Gamvar can potentially fix it permanently if you prefer. Otherwise, waiting until the painting can be varnished with your preferred final varnish is the best, most durable fix.