I first began blocking with wet/dry 220-grit 3M paper. Begin using a hard block with a touch of water running over the block at all times. When blocking, use long strokes, 10-12 inches, to ensure the area is being sanded evenly. Short strokes will create sinkholes and low spots in the finish, a big no-no. Here's where the guide comes into play. Notice after a few strokes the unevenness of the finish is clear as day, hence the need to block. Sand the area until an even finish of base is all that is left.