Just curious what you think about the latest theory about how this was painted from the daVinci authority at the Getty. He believes the sfumato technique was achieved by using glazes in a micro pointilist method to build up the shading and over many years the points spread out creating the beautiful haze. If you decide to experiment with this - please post your work as it is evolving. Very interesting blog !
I've actually been experimenting with a very similar technique, based on Leonardo's drawing technique. I'm doing a series of fine crosshatching with a small pointed sable brush and thin glazes of paint mixed with walnut oil. This allows me great control over placement and densities of color without overworking the surface. I'd have to concur with the Getty expert.