I always use sealer before paint. I prime and block multiple times before I paint a car too. Once you have the car ready, masked up ,wiped down and tacked, shoot a nice wet coat of sealer. Let it flash for an hour or so then start shooting your color. I finish my surfacer with 320 dry, sometimes 400 wet. Either way I always seal. It makes for a nice barrier between the surfacer and color. Also, the sealer I use can be mixed in various colors to help your color cover quicker. Any good paint shop should be able to mix up the color you need. Good luck.