The 21" is AFS"s Most Popular Sander! By far the most popular AFS sanders in use today. A great choice when working on street rods, muscle cars, or anywhere where nothing less then a show finish will do. You wouldn"t believe how much easier and quicker it is to straighten a panel with just six more inches of sander. Even more importantly, a lot of thought and care went into the design of it, so it gives the user an unbelievable sense of confidence knowing they"re doing the job right the first time. Made out of 8 lb. density foam, has a .015" thick base plate. Comes with 3 - 5/32" rods and ends caps. Each sander comes with a set of vinyl end caps to prevent the rods from sliding around and to prevent unwanted scratches when working in tight areas. How They Work and Why They are Unique: By adding or removing flexural rods of same or different diameters in different configurations, you change the pressure to flex-ratio. In other words, you control how much the sander flexes. These sanders will pick out lows that your hand or naked eye almost always miss. The number of rods inserted effects the stiffness of the sander. This allows you to go from sanding the fender of a 1940 Chevy to the hood of a 1965 Lincoln in a matter of seconds. Each sander comes with a set of vinyl end caps to prevent the rods from sliding around and to prevent unwanted scratches when working in tight areas. How They"re Made: The base plate on the 4.5", 9", 15", 21" and 27" sanders is made from 1075 tempered spring steel. Ranging in thickness from .008 to .020" depending on length. The base plate on the 36" sander is a little thicker, made from .060" thick aluminum, (around 16 gauge). The flexural rod chases within the sanders are made from polyethylene and the foam handle is made from the finest foam available.