Historically abrasives were made from natural materials such as sand, shark skin and shells. These might be applied directly to a surface or bonded to a cloth or paper backing before use. These materials were superseded by glass paper in the 19th century. Glass paper has itself been replaced by materials such as aluminium oxide and silicon carbide.
Modern abrasives tend to retain their sharpness longer and are better bonded to more durable backing materials, giving increased working life. Some papers (‘wet and dry’) are sturdy enough to withstand being used with water to lubricate the work and wash away clogged particles. These are used for automotive paintwork and metal finishing.
Papers are sold in sheets, rolls and in special shapes to suit various sanding machines. Rolls are usually the cheapest way to obtain paper. Some papers are ‘open coat’, ie. the grit particles are spaced apart to prevent debris clogging the abrasive. Some also have an additional coating to lubricate the paper.
Abrasive grades and purposes
The grit size of an abrasive dictates how rough or smooth it is. The grade is usually printed on the back of the paper. The coarser grades (40, 60, 80) are intended to remove material and change the shape of items. They are referred to as ‘production papers’. Medium grades (120, 180, 240) are a useful intermediate stage to remove the marks left by coarser papers or to ‘key’ a smooth surface to assist adhesion of a subsequent layer. Fine grades (320, 400, 600, 800, 1000, 1200) remove much less material and leave a successively smoother surface.
40 grit is useful for levelling hardened fillers and cutting through old paint layers. It can be used on paintwork to remove dried paint runs and other defects but the surface left behind will be too rough to give an acceptable paint finish. Rubbing down with successively finer and finer grades will be required to remove the scratches left by the coarse paper. For gloss paint 80 grit followed by 120 grit should be sufficient prior to undercoating.
If there are no major defects a rub down with 120 or 180 grit aluminium oxide paper prior to undercoating is enough to provide a key on old painted surfaces. Subsequent coats require a light rub down with 240 or 320 grit to ‘de-nib’ the new paint before another layer is applied.
Papers finer than 240 grit are used to prepare bare wooden items for finishing. Papers in the range 600-1200 are used between coats to flatten or de-nib layers of French polish, sprayed paint finishes and varnishes. Even finer abrasives, such as pumice powder, are used in the final burnishing of polished surfaces.
Wire, or steel, wool is available in a range of grades from extra coarse to super fine. These roughly equate to the range of abrasive paper grades and are used for similar purposes. Wire wool conforms to round items and mouldings better than abrasive papers.
Coarse wire wool can also be used to remove material softened by paint strippers. Stainless pan scourers sold in supermarkets can also be used for this purpose and have the advantage of not rusting after contact with water-based strippers.
Finer grades of wire can be used to apply waxes to bare wood, although they should not be used on oak as any residual metal fragments will react with the tannin in the oak, causing discolouration. Super fine grades are used to de-nib French polish and to apply revivers to polished surfaces. As wire wool is softer than glass it is useful for cleaning windows, mirrors etc.
Abrasive pads and related products
eg. Webrax, Abranet, Scotch-Brite.
Abrasive pads can be a foam-backed block with a layer of abrasive bonded to the outside, or consist of a fibre pad or net material coated with abrasive particles. They are available in a range of grades. As they conform well to curves they are useful for sanding mouldings. They can be used wet or dry and rinsed out if they clog up. They are durable and can be used on a wide range of surfaces.
Abrasive papers should be used on a block to avoid furrowing and maintain flatness. Blocks can be made or improvised where odd shapes are required, eg. sections of dowel can be used to support paper when sanding internal curves. The appearance of lines and scratches can be reduced if sanding is always done parallel to the grain and care is taken to remove any residual grit from the surface between grades.
Substrates to be polished or varnished require more steps and more care than painted surfaces. Any defects, eg. scratches from coarse abrasives, will be visible through the finish. Finishes should be thoroughly cured before sanding is attempted.
There are health hazards associated with these processes. Machine sanding of any material can produce respirable dusts which is some cases are carcinogens or respiratory sensitisers. Any paint from earlier than the mid-20th Century may contain lead. Use extraction where possible, work in a ventilated area, wear a well-fitting PP3 dust mask and vacuum clean the work space to reduce these risks.