I recommended doing the hatching on a separate layer, then erasing the extra areas. If you don't want the edges to be fuzzy, go to Brush Tip and set the Fade values to 1.00. You can get various grid effects this way. To nudge the hatching, you can tweak these two values. Origin X and Origin Y: The hatching has a fixed location, painting acts as though you're revealing the existing hatching underneath.