Drawing Fixative used on canvas

[Mike Dodson]

I have all of Sanden's books that he has published over the years. He speaks a great deal about his methods and materials. One of his methods that is particularly interesting is that on his commissioned work (not demo's from life) he produces a detailed drawing in graphite directly on a white canvas. Afterwards he sprays the drawing with fixative to prevent smudging ( I assume that is what it is for). Is this layer of spray between the canvas and oil paint stable? I wouldn't even question this if not for the fact that he is one of the most well known and highly acclaimed portrait painter's of our time and must have discovered that this method does not harm the painting.

I would really appreciate someone with knowledge in this area to let me know what is going on here.

Thanks!