The accompanying picture shows what you don’t want to happen when you’re sanding a finish. The little brownish lumps, called “corns,” are finish that has been melted by the heat created during sanding, and they are large enough to leave much deeper scratches in the finish than those created by the sanding grit.
These scratches will then have to be sanded out to keep them from telegraphing through the next coat.
Some finishes cause corns much easier than others. These include oil-based varnish, lacquer and shellac. Oil-based polyurethane, water-based finishes and high-performance (catalyzed) finishes don’t cause corns as easily. Sanding sealer also resists corning because it powders so easily when sanded.
Though it’s often difficult to totally avoid corns when sanding, here are three suggestions: let the finish dry longer before sanding, use a stearated (dry-lubricated) sandpaper, and use a light touch.
Clearly, a finish that isn’t totally dry will melt quicker under the heat created by the sanding than a finish that is fully cured.
Stearated sandpaper is coated with the same zinc-stearate dry lubricants that are contained in sanding sealer. The stearates improve the powdering characteristics and resist corning.
Reduce the pressure when sanding. Pressing on the sandpaper creates more heat.
Image: Corns on wet/dry sandpaper