If i understand you correctly, you are trying to soften the sharp edges of the plywood. So in essence a small chamfer. Try using a small blockplane with a sharp blade. Hold the plane on the edge of the plywood, at 45° to the face and edge, and skew the plane 45° so that the face fibre's are cut before the edge fibre's, the blade will slice the fibre's by pushing them down and into the board. You will get a very clean cut with no tearout. You need only take a very fine shaving. You can take several passes and make adjustments to the plane angle to make a small roundover if you don't want a chamfer. You can then finish with 240 grit sandpaper if you wish.