MITRA Forum Question Details

​A watercolor-like wash is not really recommended HOWEVER if you are applying fatter paint on top of ALL sections that possess this thin layer then you will likely be fine...you might try adding slightly less solvent and "thinning" out simply via manipulation of the brush. This may mitigate the strike-through that you are seeing on the reverse of the canvas. The latter tends to occur when the size and/or ground is not adequate and/or you are overly thinning your paint with diluent. OMS is completely appropriate to use for oil paint as long as you are not using certain additives such as natural resins or other materials that are not soluble in OMS.
As for this question: "If this thinned layer leaves the ground with much tooth available for thicker paint to adhere to, would delamination problems persist." We are not sure what you mean by too much tooth. In general tooth implies that your ground is textured enough to adequately allow for adhesion of subsequent layers. 
And this question: "And I've come across the idea that oms evaporates fully and thus doesn't alter the lean-ness of the paint once it's gone, is this true?" Solvents that leave no residue do not, in theory, alter the lean-ness of the paint layer. However, one could still potentially overly thin an oil paint layer to create an underbound surface to begin with.

When overly-thinned paint yields a dull, lackluster appearance, it's because the viscosity of the paint vehicle has broken down. When this happens, the envelopment of each pigment particle is lost, and along with it both binding power and the essential optical function of the oil.

"And I've come across the idea that oms evaporates fully and thus doesn't alter the lean-ness of the paint once it's gone, is this true?"

Kristin, I was thinking the same thing you mentioned in your reply to this question. In theory, if the thinned paint were applied to a completely impermeable material like glass, one would expect the oil-to-vehicle ratio ​to remain the same after all solvent has evaporated. Since a primed canvas is porous and absorbent, however, and because staining appears on the back of the canvas, it seems reasonable to assume that some of the vehicle has been taken up by the ground and support material.

Probably, it would be difficult to calculate the precise percentages that you are looking for. There are just too many variables. Each pigment has a different oil absorption requirement. This is the minimum to create a workable paint. However, manufacturers will likely have to include a bit more than this to make a paint that does not require onerously long milling as well as a paint that can be thinned without causing the issues you mention. Each manufacturer will do this to a different degree. The highest-grade paints with the least amount of stabilizers/fillers could probably be thinned to a different amount than those containing far more oil and are chock full of aluminum stearate. Student grade paints, a different amount. Very well made hand-made oils an even different amount. The short answer is no.  It is likely adequate to just never thin your oils more than a thin cream or whole milk consistency.

Imagine paint as analogous to a brick wall, with the pigment as bricks and the oil as mortar. Too much water in the mortar leads to lack of cohesiveness, weakly bound bricks and ultimately a failed wall. In paint, excessively thinned vehicle cannot maintain envelopment of the pigment, so there's nothing to stick the particles together. When the solvent evaporates, there might be the same amount of materials, but just like a failed wall where all the bricks and mortar are still there, cohesiveness and orderly structure are lost.

​Little to add that has not already been covered by Brian, Kristin, and Matt. However one way to test if you have over-thinned paints is to use a q-tip and some OMS after the color has thoroughly dried - say two weeks - and then see what degree of color lift you get when gently rubbing an area. If color comes off easily, that's a problem. If it takes some time or work, then at least one can assume that subsequent layers might lock them down.

Also, keep in mind that the amount of thinning and the appearance of a matte film will be pigment dependent as well - with very small particle pigments requiring more oil to pigment ratio to fully wet out and so likely more tolerant of thinning. Larger particle colors will by contrast appear matt more easily.