Sealing the MDF before priming was something of an unclear process. After a bit of research, one of the methods I found used automotive 2K urethane primer and skipped the sealer. I already had access to the urethane primer, so I tested it on a scrap of MDF with good results. A couple of coats of primer later, and the pulse rifle's really looking good! There are a few spots of fuzz from the bare MDF and various pinholes and flaws that need filling with putty. Using 180 grit sandpaper, everything seemed to sand out really smooth.