When sanding hardwoods, the aim is generally the smoothest surface area possible, but this isn't the situation when sanding an exterior deck surface. For sanding deck floorboards, use sandpapers with grits of 60 or eighty, but no rougher than that. The power sanders accustomed to sand a deck is available http://messiahfbtlb.bluxeblog.com/29189994/how-to-do-two-coats-of-stain-on-a-deck-an-overview