I coat the entire sheet, and then place a mask made out of card stock with the size of the final print cut out of the center, in between the negative and glass. When you develop it, the masked off area washes clear because it was never exposed to light. It also helps to reduce the risk of Newton rings from the glass laying directly on the negative, though I haven't really seen the effects of any Newton rings during alternative process contact prints, so I don't usually worry about them.