I've done a bit of research about this and as far as I can tell it is tllatoy fine to paint as many layers as you wish over the top of each other as long as they have the same amount of oil content added. And this would be the same for walnut alkyd medium.The main rule about fat over lean is to never do lean over fat. That is, never put more solvent in paint over a layer of paint with less solvent. Adding more oil to a paint layer makes it more flexible and slower to dry while more solvent makes it faster to dry but more brittle. So the main thing you want is the layers on top to be as flexible or more flexible as the layers beneath, as well as making sure they dry at the same speed or slower. Because we are talking here about painting without solvents the rule is a bit more flexible just make sure you either keep the amount of walnut alkyd medium the same in each layer or increase it.I don't think it would be a good idea to use a lot alkyd medium for the beginning layers to make them dry quicker and then use little or no medium on the subsequent layers. These layers would be slower drying and more brittle which could potentially lead to cracking problems in the future.