I am finding that after a period of about 1 year or more, surfaces where I have applied very thick areas of white oil paint have brown oil spots, as if the oil in the paint is seeping to the top. This may be due to the low quality of paint I’m using: Winton – or perhaps the kind of white: Titanium. In these cases I have not mixed any mediums into the paint. What is the cause, and can it be repaired? Can I simply paint over the surface with a higher quality white, and will that last?
I’m wondering if this is the beginning of what is called phase separation. It usually happens when oil paint ground in semi-drying oils like safflower and sunflower oil (Winton titanium white is ground in safflower oil) is applied in thick layers (metal soaps seem to be an important contributing factor too).The actual alchemy of what causes this is beyond my pay grade but at some point orange-brown oily exudation (likely binder that has not fully oxidized) starts to break trough the paint layer (and even appears in on the back of the canvas).You can read the study by Dr Jaap Boon that was done as part of a law suit (against Schmincke by Dutch artists Frank van Hemert) here: https://www.jaap-enterprise.com/wp-content/uploads/2016/06/Boon-Hoogland-ICOP.pdfA Dutch news item with better visuals can been seen here: https://drive.google.com/open?id=1I0ZB4irtoPZ9KxADJiCRNm3Efph3dh52
p.s
My apologies for the bad formatting; I pasted from a text editor and the forum software doesn’t seem to allow editing.
Also, the item about Frank van Hemert starts at around 20:00.
I cannot say that I have experienced the exact phenomenon that you describe. I have tried to always use higher quality oil paints for anything other than the most trivial color assignments. W & N does freely admit that Winton is their student line but even so, this should not be happening. I generally think of lower grades of paints having greater proportions of fillers and stabilizers that, I would think, could make the pigments less likely to sink and have a surface skin of oil, which would yellow. This could be more than offset by the lower pigmentation inherent in student grades but still this seems surprising, I wonder how you applied the thicker strokes of paint; by brush or palette knife. I have always been struck by how different the surface effects of the same paint appear when using these two different tools. This seems to go beyond the simple smooth surface left by the knife as opposed to the textured artifact of the brush but this is a bit too esoteric for the present discussion.
Anyway, I am sorry that I cannot give you answers that are more helpful. Hopefully, some of our other moderators can answer with more coherent and cogent remarks.
Finally, I would say that you should certainly paint over the offending passages with a better quality white, preferably with a highly pigmented, artist grade titanium white. This would likely mean that you would apply a more pigment rich paint over a fatter paint but it is appropriate in this case given the unsightly surface. If it were me, I would lightly abrade the surface of the offending paint with some very fine sandpaper (given that this is titanium white and not lead white or a mixture of titanium and another toxic pigments, ignore this suggestion if this is not the case). 320 or 400 grit should work. This will promote adhesion, which otherwise could have been compromised due to the slick surface skin of yellowed oil.
I cannot speak to that, but have sent this question to Dr. Boon to see if he has any comment.
