I'm plastering my first drywall and have been given advice from a friend who as done it a few times before. He believes I should not sand down the mud (a.k.a. jointing compound) between coats, but I think I should. I don't think the first layer is as smooth as it should be and I think getting any little bumps out now before the next coat would be a good idea. Surely the next swipe of the knife is going to catch on them and leave elongated ridges or scratches down stream.