Thank you for the advice. The reason I thought thinning the paint with mineral spirits was a good idea is that when I mixed the medium recipe it had some in it and I knew that some painters use it to thin all of the time. I'm glad you told me about the walnut oil. I would have had to mix some more of Marc's medium and I didn't really want to since it didn't seem to come out right. I will try the walnut oil.