The term 'sandpaper' is used these days to cover abrasive grit on flexible backing sheets used to smooth many types of material. True 'sandpaper' (i.e. backing paper covered with grains of sand) is no longer available commercially but has been replaced by backing sheet covered with glass, aluminum oxide, silicon carbide, garnet or other specialist grit. Generally the terms 'sandpaper' and 'glass paper' are used generically to cover all types of grit attached to a backing sheet.

Each type of grit has different characteristics which make each suitable for specific applications, an understanding of the types of grit is essential so that the right type is chosen for a particular job.

In simple terms, the abrasive grit is fixed by an adhesive to a backing sheet - the type and size of grit, the type of adhesive and the type of backing material all have an effect on the suitability of an abrasive for a particular job.

Various sizes of grit are available for all types of grit material, the size is referred to by a number which represents the number of holes per linear inch in a sieve screen - they range from 40 (very course) to over 400 (very fine). Good quality abrasive will have universal sized grit. The size of grit is used to classify the abrasive by 'grade' as follows:

The individual sheets of abrasive are normally marked on the reverse with the grit size (i.e. 120) and/or with the grade (i.e. medium).

Most commonly, abrasive is available as 'closed coat' (i.e. completely covered with grit) however 'open coat' (where only 50 to 70 per cent of the backing is covered) is available for most types of abrasive paper although only a specialist supplier may stock them. Closed-coat abrasive paper cut faster, but are more likely to clog (i.e. the intergrain space become clogged with the waste from the material being sanded) than open-coat sandpapers.

Three types of backing materials are commonly available:

• Ordinary paper - Although relatively a cheap backing, ordinary paper is an adequate material for most sandpapers. The quality of paper varies depending upon the intended method of use (and price); sandpaper supplied for use with power tools generally has a tougher quality of backing paper than that sold for hand use.
• Waterproof paper - Water proof paper is essential where the sandpaper is to be used with a lubricant. The back of this type of paper usually has a darker, glossier appearance.
• Cloth - Generally used where a high degree of flexibility is required when using the sandpaper.