Sandpaper: the specifics

A range of adhesives suitable for wood, left to right: 40 grit aluminium oxide open coat, 40 grit red garnet; 60 grit brown emery, 60 grit yellow aluminium oxide closed coat, 80 grit red garnet, 120 grit black silicon carbide, 120 grit white aluminium oxide and 240 grit black silicon carbide.

Words: Philip Ashley

The first abrasives were made in 13th century China from crushed shells and sand bonded to parchment with a natural gum. These days, you won’t be using shells, sharkskin, sand or glass to finish your woodwork, you'll be using modern materials designed to do a very specific job. So let’s take a closer look at coated abrasives, more commonly referred to as sandpaper. Coated abrasives are supplied by all hardware stores in a wide range of different backings, abrasive grains and grit sizes. You just have to have an idea what you’re looking for.

Coated abrasives come in sheets or rolls for hand sanding but are also available in many other forms. Whether you need a wide belt or a bobbin for a sanding machine, or sheets to fit your power tools or for hand sanding and finishing your work, the principles of coated abrasives remain the same. Remember too that there are at least half a dozen major manufacturers of coated abrasives and they all produce the same range of products. Some of it is coloured for sales reasons but the design and technology of the abrasive product is identical.

Backing

The two things about coated abrasives important to the woodworker are the backing material and type of abrasive. The backing material is usually paper or cloth but could be polyester or a cloth and polyester mix. The backing itself may contain a code with a letter indicating it’s ‘weight’ or thickness. Starting from A to Y the weight of the backing goes from light paperweight to very heavy cloth. Lightweight backings have a high degree of flexibility and can be used for intricate sanding of curved parts. Heavy backings are more aggressive and less flexible but are stronger and more suitable for larger grits (40 to 80) when heavy removal of stock is required.

Washable sponge abrasive blocks are reported to last five times longer than sheet abrasives of the same type and tend to resist clogging.

Abrasive types

The type of abrasive will vary according to the job it’s meant for. For woodwork the most common abrasive grains are aluminium oxide and silicon carbide and these materials have been used for decades. For hardwoods, use aluminium oxide; it's hard wearing and tough and out-performs most other abrasive materials.

Softwood is more fibrous and for this, silicon carbide is the best abrasive. It's very sharp and cuts very fast with only light pressure. Silicon carbide is also better for smaller grits such as wet and dry paper because it leaves finer scratches. Garnet is a natural abrasive and is also suitable for softwood, especially pine.

You used to be able to tell the abrasive type by the colour but not anymore. Aluminium oxide can be white (98.5% pure), grey (97% pure), brown (95% pure), green and yellow. Silicon carbide can be green (highest purity), white, but more commonly, black. Garnet is usually red or brown and if you come across flint sandpaper, it will be white.

Open and closed coat papers

The grains on your abrasive will be spaced as open coat or closed coat paper. With open coat paper, 50% to 75% of the paper or cloth will be covered by abrasive. This is to reduce the effect of loading or clogging of the paper when sanding resinous wood. Closed coat paper has the entire surface coated with abrasive. This is the more common type and gives high stock removal and long life.

Grit sequences

On the back of the abrasive sheet you may find information on the paper weight and type of abrasive but don’t count on it. The only thing you can be certain of is that the grit size will be stated.

Before using any abrasive it’s important to plan your sanding sequence. This will set the grit of the abrasives you need to use for the finish you need. For heavy stripping you’ll need to start with 40 grit abrasive. For rough sanding, start with 60 grit. Light sanding needs 120 grit and fine finishing needs 180. The grits actually come in grades closer than right up to 2,500 though 800 is about the limit for fine finishing wood. As a general rule of thumb, you should never skip more than one grit size or you will not remove the scratches made by the coarser grit.

Why is this? When sandpaper is made grains of abrasive are first glued onto the backing with either an animal based or synthetic adhesive. This is called the ‘make’ coat. After the glue is set, a second coat of adhesive is applied. This is the ‘size’ coat and fills the gaps between the grains to a pre-determined level, providing a precise height of grains above the adhesive (see diagram above). When this paper is used it produces scratches on the workpiece to the same depth as the grain height above the size coat. Only the next two (higher) grit sizes will remove those scratches and replace them with smaller ones. Do this enough times and you will get a surface with fine scratches that to the naked eye looks very smooth.

Storage

Like all your woodworking tools and equipment, your coated abrasives should be stored correctly. High humidity may cause the bond to soften and this may release the grits when sanding. The abrasive sheet will also tend to clog more easily. High or low humidity may also cause the backing to shrink or stretch more than the adhesive and the sheets may cup. Newer synthetic backed product is not affected as much by humidity but in low temperatures may achieve what is known as ‘cold set’ condition and should be allowed to come to room temperature before use.

Your choice

So what are you looking for when you buy coated abrasives? If you were sanding a car body you could be quite specific, but wood is very different. As an example, an American hardwood is about the same density as an Australian softwood so you may have to experiment to get the right solution. If you’re a woodturner you may want a flexible backing paper or cloth. If you use wood that clogs the abrasive, try an open coat paper. If you work with hardwood you need aluminium oxide, and if you work with softwood you need garnet or silicon carbide grains.

Lastly, you will need a range of grit sizes to ensure your work ends up as smooth as you hoped, remembering that you can only jump one grit size. It’s a good idea to talk to your supplier about the abrasive you need, and if you’re buying online, avoid cheap Chinese-made papers and ask questions if you can. Abrasives are tools and just like all your other workshop equipment, proper selection, care and use will produce satisfying results.

Philip Ashley is a long-standing contributor to Australian Wood Review magazine. See above, top right for links to a few of his other articles.