The BEST SELLING Mr. Surfacer is a type of surface primer that also serves as a fine-grained filler, primarily used in model making and sculpting. It's designed to prepare surfaces for painting by filling in minor imperfections, seams, and scratches, as well as providing a uniform base for color coats. The number designation (e.g., 500, 1000, 1500) indicates the coarseness of the filler, with lower numbers representing thicker, more robust fillers.