As with the preceding bodywork, block sanding is a job that requires learned skill. We once again primarily used hard, flat sanding boards for the block sanding, both the long board and a quarter-sheet short board. Our preference in block sanding is to start out with a fairly aggressive cut, using No. 180 dry paper. The coarser paper will level much better than a finer grade, which will tend to "float" over flaws rather than cut. The guidecoat should be evenly wiped away as the sanding progresses, showing a level panel. Dark spots where the guidecoat remains indicates a low area, meaning the panel needs more sanding to get it perfectly level. If the sanding starts breaking through the primer before the low spot indicated by the guidecoat cleans up, the flaw is too deep and will need additional attention. Here a judgment call is made whether to use a filling glaze or another primer application, depending on the extent of the defect.
After completing the block sanding with No. 180 grit, and any minor flaws are filled, we like to go in with a second application of surfacer. At this point, the surface should be nearly perfect and level, and the second application of primer is for texture, to fill the relatively rough sanding scratches. First, the car is detail sanded again, this time with No. 320 dry. Going over the surface with the No. 320-grit paper will reduce the sanding scratches that will need to be filled in the second primer session. Before the second primer application, we re-applied seam sealer as required at the hinge box and jamb seams, which had been removed when the car was stripped. The car is once again solvent cleaned, air dusted, and wiped with a tack rag. Because we were dry sanding, we did not remove the masking from the first primer session. Some painters prefer wet sanding, which requires removing the masking job and retaping. The second primer application is lighter than the first, since little filling is required. Two coats to give even coverage should be sufficient, followed by a fogging of guidecoat.
After the second primer application, the car is wet block sanded with No. 400-grit sandpaper. This final block sanding will provide additional leveling to the panels and a smooth silky surface for the finish painting to come. You know that it's perfect if the guidecoat washes away evenly in the final wet-sanding process, and the wet primer reflects a dead-level surface. This is the surface that great paintwork is built on and makes all the effort worthwhile.