I scrape them with a razorblade scraper. You can use a bristle disc on a die grinder but some material may get down the ports and into a cylinder if a valve is open. (not the best idea) I always silicone the corners where the gaskets meet the china wall. Poke a small screwdriver down the bolt holes in case some go into water. Those need teflon or silicone on the bolt threads. Depending on the gaskets, some guys put silicone around the coolant passages especially if you are using the stock type embossed steel gaskets.
