… my best guess would be that you’re not getting enough pigment into those areas. Getting back to your palette more often may solve that problem. -Harry

These spots look to me like the canvas showing through. Maybe when you first paint and the paint is wet you do not notice that you have not covered all the canvas.
It will help if you mix a few drops of Linseed oil into your dipper of (I assume you are using pure Turpentine not substitute) Turpentine. One other point is that it is not always necessary to use a medium at all…only if you feel the brush is dragging or it is not coveriing well.
If you do buy some Linseed oil try painting a few drops of pure oil onto the spots if it is blooming it will disapear…although bloom is more of a grey haze over the surface than spots.

Gina

I recently painted an impasto landscape. While it was wet, I didn’t notice any white spots, due to the reflective sheen of the wet oil paint. There were alot of little reflective areas,throughout the painting. However, after the painting dried somewhat, and the sheen diminished, there were little spots not covered with paint throughout the canvas. I simply used a very small round brush, and place tiny dabs of paint into these “holes”. Normally, I tint my canvases, but with this painting I didn’t tint. Tinting the canvas via tinted gesso, has helped me to avoid seeing these little spots, on alot of my paintings. I recently went to see the Van Gogh “Wheatfields” exhibit at the Toledo Museum of Art. I had the opportunity to closely observe these paintings, and found what appeared to me to be many of these little uncovered “white spots” throughout most of the canvases. One of them was almost shocking to me, as it appeared to have many areas of the canvas, with no paint on it. It almost looked as though Van Gogh was careless. I remembered a quote, by Van Gogh after someonehad accused him of painting too hastily. He responded by saying that (not a direct quote) if they thought that he painted too hastily, then they has in effect looked at the painting too hastily. It made more sense, to me, but I still feel uncomfortable about areas of a painting where canvas is showing through. I have never observed these areas on the Monets, or Degas but have also seen these uncovered spots on paintings by Cezanne, and Pissaro. I like all of these artists works, however, white spots and all. I buy paints with linseed oil, as I have found that the ones with safflower seed oil dry slower for me. I also like the smell of linseed oil better. It’s a matter of preferance. The white colours that I buy all have safflower seed oil as a vehicle, because it doesn’t yellow as it dries, as can happen with linseed oil. This is not much of a problem with other colored paints, because the yellowing is miniscule, but more noticeable with whites.

Jim