If you use the Paint Roller tool with a full-size drawing and the paint leaks, Zoom In is very useful for patching the areas where the paint has leaked through. Using Zoom In to edit one pixel at a time From the View menu, choose Zoom In. The cursor changes to a movable rectangle. Place the cursor over the area you want to magnify. Click the mouse button.
Paintbrush magnifies the portion of the drawing enclosed by the rectangle.
In the small box that appears in the upper-left corner of the drawing area, you can see how changes you make to the magnified area look in normal resolution. Point to pixels and click them, using either mouse button. The left mouse button changes a pixel to the foreground colour. The right mouse button changes a pixel to the background colour. Dragging changes a wide area of pixels.