Sandpapering Tips

Sandpaper is an abrasive-coated flexible sheet used in woodworking for giving a fine or rough finish to individual work pieces, large surfaces such as flooring, or smaller local areas such as a sticky window frame. They are used both by hand and in power tools, and are available in a number of different materials.

In each of the following types of sandpaper, there are three main grades; coarse, medium and fine, and each of the three grades are subdivided into finer categories, for example, Medium 3A. The grades measure the size of the abrasive particles, and also the spacing of the particles.

Widely spaced abrasives, called “open cut”, are useful for materials which tend to clog up the grains easily, like paint. The more closely spaced grains, called “closed coat”, cuts the surface rapidly but will also clog more quickly and need changing more often.

Glasspaper

This is the least expensive abrasive available. It is a light yellowish color, and often is mistakenly thought of as sandpaper, for the visual resemblance to sand. Although sand is actually no longer used as an abrasive in any types of abrasive paper, the term sandpaper is what everyone still calls all of them. Glasspaper is used for roughly finishing lumber, and wears quickly. It is not suitable for fine finishes.

Garnet

A naturally occurring red mineral, finely ground, backed with paper. Used for hand finishing all lumber types, both softwood and hardwood. It is harder than glasspaper. Available in finer grades, it is to be used dry.

While the pure crystals of garnet are used for gemstones, garnet in sand form is a good abrasive, and a common replacement for silica sand in sand blasting. Mixed with very high pressure water, garnet is also used to cut steel and other materials in water jets.

Emery

A natural material fixed to either paper of cloth backing, it is used mostly for metal finishing. Strips of cloth backed emery are especially good for hand sanding round stock, such as tubing or rod, but useful also in curved surfaces. Paper backed emery is good for flat surfaces. It can be used with water as a lubricant so that the work piece does not become overheated and distort.

Silicon Carbide

A synthetic material that is harder than emery, fixed to waterproof paper. It is also known sometimes as Wet and Dry Paper, because it can be used I either state, to finish paint or metal. While working on paint, the particles of paint and water form a slurry mixture under friction; the slurry should be wiped from the work piece while it is still wet other wise it will be difficult to remove. The paper itself should also be frequently rinsed with water to prevent paint from clogging it’s grain. On bare wood, silicon carbide paper should only be used dry.

Aluminum Oxide

This synthetic material is available with cloth and paper backing. It is used for machine sanding of metal, plastics and lumber.

Tungsten Carbide

Mainly used in machine sanding, this is the hardest available abrasive material. It is available in thin metal strips or disks, depending on the machine type.

Tungsten carbide is four times harder than titanium, and is twice as hard as steel. It is absolutely un-scratchable, and has been used for decades in industrial applications such as cutting tools, mining machinery, and rocket engine nozzles. Its extreme hardness makes it useful in the manufacture of abrasives and other cutting tools.

Sanding Blocks

For hand sanding, using sandpaper on it’s own can be difficult to work with, as the paper will follow any dips or bumps in the work surface; this will produce uneven finishes. Wrapping the paper or cloth around a sanding block keeps even pressure on a larger area of the work piece. Blocks are sized to used be easily held with one hand.

Sanding blocks are available in various sizes and can be made of plastic, rubber, cork or softwood. One quarter of a regular size piece of sandpaper is perfect for wrapping around a sanding block. Do not cut the paper with scissors; instead, tear it over the edge of a workbench. Fold the sandpaper sheet in half. Then, hold one half of the paper firmly down on the bench then pull the edge down to tear off the other half.

Wood should be sanded along the grain direction. Any cross-grain scratches, though they may not be apparent in the unfinished wood, will be highly visible and exaggerated after varnishing or painting. Do not angle a sanding block surface against the work piece, otherwise you will round off the edge of the wood; keep the block flat on the surface.