When building a street rod, few things are more disappointing than working hard and spending money on some aspect of its construction only to have the results fall far short of your expectations. In some cases minor flaws are only obvious to those who know their cars intimately and will never be noticed by anyone else, but then there are those that seem like they could be seen from space—and those are usually in the paintwork, or more accurately the surface below.
While it's obvious that laying down paint is more complicated than pointing the spray gun the general direction of the car and pulling the trigger, regardless of how well paint is applied it will never make bad bodywork better. Paint a corncob and it's still going to look rough, the reason is the paint will not level the surface but follow it. Those spectacular paintjobs we've all seen on the pages of Street Rodder and in person look that way because they are extremely reflective and a smooth surface is more reflective than a rough one. And block sanding is the secret to a flat surface.
Block sanding is a term that most of those in the hobby have heard, however it is still confused with hand sanding, which should be avoided. The problem with hand sanding is that pressure is unevenly applied to the sandpaper; look at a handprint on a flat surface to see what we're talking about. (Editor's note: The staff agrees with Ron as his hand print oddly resembles a paw.) We've said this before but fingers are for scratching, hands are for holding, and blocks are for sanding.
One of the common questions about sanding in general is what grit sandpaper to use. Coarse grits labeled in the 36-80 range are used for removing old paint, filler, and rust. Grits between 100 and 320 are used to smooth body filler and primer surfacer. To prepare primer for paint, 400- to 600-grit abrasives will smooth the surface while still giving the paint something to "bite" into. Finally, extremely fine grits, such as 1,000 and up, are for color sanding prior to buffing.
Along with grit, two additional terms that will be used to describe sandpaper are open coat and closed coat. Basically this describes how tightly packed the grit is on the backing material. Open coat abrasives have the grit spaced further apart, which helps reduce clogging when soft materials are being sanded. Closed coat means that the grit is packet tightly together and all of the backing is covered. Closed coat sandpaper is more aggressive simply because there is more grit on the backing.