20. Sanding the primer is critical to a distortion-free finish. Use long, hard boards rather than flexible rubber blocks, which follow distortions in the panels, and whack the surface hard with wet 220-grit paper or dry 150-grit. Coarse paper quickly whittles down the high spots without riding into the low areas. Where trouble areas appear, work the panel as far as the primers material thickness will allow, but stop sanding if it cuts through to the layer below.