Sandpaper is a tool that consists of a sheet of paper covered with sharp particles. It is used to strip, smooth, and polish surfaces, usually wood. The particles are commonly made out of aluminum oxide, garnet, silicon carbide (for hard materials), or other ceramic materials.

To use sandpaper effectively, it is important to start with a low grit and switch to progressively higher grits. "Grit" is a number assigned to each sheet of sandpaper representing its coarseness. Or more precisely, the approximate count of particles per square inch. For example, here are the common names of sandpaper based on the grit:

40-60: Coarse
80-120: Medium
150-180: Fine
220-240: Very Fine
280-320: Extra Fine
360-600: Super Fine

See also: tool

Paper with sand attached. Sand paper.

Except not sand anymore. Or ever, really. Garnet; sharkskin; glass. That's what is and has been sandpaper. Abrasion.

Lordy, the things you will know about sandpaper by the time we're done.

The first recorded instance of sandpaper is in thirteenth century China. The Chinese bonded shell fragments to parchment with a natural gum. They're also the ones who figured out sharkskin would do the trick.

Before our modern flint and garnet and aluminum oxide came around, people made sandpaper out of pretty much anything that would scrape. Garnet is a gemstone. There is jewelry with garnet in it. Polish it up and cut it, it's frozen fire; grind it up and stick it to paper and it shines wood.

Sandpaper took a bold step forward in the US in 1816 with 3M's introduction of waterproof sandpaper.

Springfield, Vermont's own Isaac Fisher, Jr. won a US patent for sandpaper in June 1834. Eighteen years after waterproof sandpaper.

The basic premise of sandpaper is the same as it was 700 years ago, but the materials used to make it have evolved considerably. Different materials are used for different purposes. Since now we sand things like metal and do so under new conditions, like high pressure, shells on parchment just won't cut it anymore.

• Aluminum Oxide is the most common sandpaper today. With heat and pressure the grits fragment, creating new sharp edges. This fragmentation makes it self-renewing and longer-lasting.
• Garnet wears out more quickly than Aluminum Oxide. This isn't necessarily a bad thing — Garnet sandpaper with the same grit as Aluminum Oxide will leave a smoother surface because the grit develops more dull edges.
• Silicon Carbide is harder than both Garnet and Aluminum Oxide; it's used for harder surfaces, such as metal, paint, and fiberglass.
• Ceramic is the hardest material of them all, and the most expensive. It's used almost exclusively with rough grits on motorized sanders to strip and level wood.

Grit refers to the roughness of the paper. The grit number is a representation of how many abrasive particles are in each square inch: to illustrate, 80-grit is way rougher than 600-grit. Like comparing gravel to powder. And like materials, different grits have different uses.

Open and closed coat refers to the presence of gaps between cutting grits. Open-coat contains gaps; closed-coat does not. Open-coat does not clog with sawdust; 40 to 60 percent of the surface of an open-coat sheet of sandpaper is made up of these gaps.

Usage

Sandpaper makes a good painting surface — Joan Miro found that the roughness of sandpaper gave applied paint a smooth look and created bold shapes.

You can use it as a musical instrument. Leroy Anderson put together a sandpaper ballet.

You can use it to finish wood also.

Now, I'm assuming you're sanding by hand. Bare-bones work. Maybe you're using a sanding pad if you've got a large surface. No random-orbit hand sanders; no belt sanders; definitely no drum sanders. Get your hands dirty, lose a few layers of skin.

The wood is darker if it comes from the core of the tree, lighter if it comes from nearer the bark. A bit like dark meat. Trees darken inside with age. Wood grain is a cross-section of the ribbons of flesh accumulated over unknown decades. Wood is flesh; grain is its map.

First, always sand with the grain of the wood when you're polishing. Going sideways or diagonally will leave a rough surface. Sand evenly, lest you leave tiny dips that will show in the light. Don't press too hard or you'll scrape. The object is to smooth the wood and blend imperfections, including those in the wood grain.

The phrase "Going through the grits" refers to using progressively finer grits to smooth wood. You'll never use a single sheet of sandpaper. For fine woodworking you'll typically start in the 120 range and work your way up to 200 or 220. Starting with too fine a grain will not only create more work for you (it's hard to smooth rough wood with a sheet of 200-grit) but it will also likely seal the surface, preventing finish from bonding effectively. Work up slowly: 100 to 120, 120 to 180, 180 to 220. If you're working with closed-coat paper, check it frequently to make sure it isn't clogged with compacted sawdust. When paper seems as though it's been used up it's frequently just gummed up with wood powder and needs a good slap or two against your thigh.

When the wood looks like glass, you're ready to add your topcoat.

Personal experience

WoodZone
www.woodzone.com/articles/sandpaper/

Sawdust Making 101
www.sawdustmaking.com/Sandpaper/sandpaper.htm

Wikipedia
en.wikipedia.org/wiki/Sandpaper

Sand"pa`per (?), n.

Paper covered on one side with sand glued fast, -- used for smoothing and polishing.

© Webster 1913.

Sand"pa`per, v. t.

To smooth or polish with sandpaper; as, to sandpaper a door.

© Webster 1913.