How To Use Abrasives

Abrasive papers, which are available in ranges varying from very coarse to very fine, have coatings such as granules of glass, emery, flint, aluminium oxide, silicon carbide and garnet. Sandpaper is a misleading term often given to glasspaper; abrasive papers are in fact used for ‘sanding’ or smoothing down a surface, but they do not contain sand. With all abrasive papers the procedure is to start with a coarse paper and work down through the finer grades until the required finish is achieved. A coarse abrasive will only give a coarse finish.

Backing paper

When using abrasives on any surface, there will be friction; this will increase with the coarseness of the abrasive you use. It is therefore important the backing paper holds together; as a general rule the backing is made thicker (or heavier) as the grit size increases. A larger grit size will mean increased friction and so a heavier backing is needed to prevent tearing.

The backing carries a number, or grit size, and also the simplified classification of, for example, fine, extra fine and medium. Low numbers indicate a coarse abrasive and high numbers correspond with fineness. Industrial users prefer to specify coarseness in terms of grit size; a typical range available in DIY shops is from grit size 40 (coarse) to 200 (fine or flour). Very hard abrasives such as silicon carbide paper come in the grit size range of 220 and 800 (fine and extra fine).

Cloth backing is used for greater flexibility, especially for heavy duty work and working metal. Fibre backing is used with industrial power sander discs because it holds its shape well when used at speed. Flexible foam sanding blocks come in various grit sizes; these are washable and avoid the need for a cork former (or sanding block).

Open coat

This type of abrasive paper is made so the abrasive grains are separated by pre-set distances; this means only 50-70 percent of the paper is coated with abrasive, allowing abraded material to clear and preventing clogging.

Types of abrasive

Abrasives for use in the home can be classified as natural or man-made. Available in a wide variety of forms, each type has particular qualities which make it suitable for use on wood or metal or both.

Garnet

This is a natural abrasive of crushed semi-precious red stone. It has hard, sharp-cutting edges and is

Wire abrasives

These include wire wool and wire wheels and cups (power drill attachments) and are used for general cleaning-up on a variety of surfaces. Wire wool is used for flattening coats of paint or varnish on metal and wood before applying a further coat. Wire wheels, brushes and cups can be used to remove paint, rust and grime. The cost of these pteducts varies according to the type. They are durable, but prone to serious rusting.

Hand finishing

Where possible use abrasive paper with a cork or wood sanding block; this helps give an even finish since it holds the whole of the surface of the paper against the work. If you use the paper in the palm of your hand, it will only abrade the area where your hand is pressing. If you have to smooth down a shaped edge or a moulding, cut a matching shape block out of cork or wood and use the paper wrapped around it. To get the block to match the edge, cut it roughly to shape; put a piece of abrasive paper (rough side outwards) on the moulding and rub the block shape over it.

Finishing metal

Once the metal has been roughly shaped, select your abrasive and the type of backing — flexible (cloth) or paper. Work with an even pressure in straight lines along the length of the work. If the work is an irregular shape, the general rule is to work concave areas with convex formers and convex surfaces with the shoe-shine method.

A fine abrasive will give a smooth finish to a flat surface; but for an extra smooth surface, you can dye the surface with engineer’s (Prussian) blue to highlight the imperfections and abrade the high spots until they are removed. On soft metals work through the grades of abrasive and give a final polish with a liquid abrasive or the finest grade of abrasive paper available.

Car bodywork

Be very careful about preparation; clean the whole area with a wire brush, wire wool or a coarse abrasive. After filling the dent or scratch, a disc sander with either a tungsten carbide or aluminium oxide disc will remove the worst irregularities. An air-powered orbital sander with a wet silicon carbide attachment is even more effective. Finish the surface by hand with wet silicon carbide paper; work over the metal, paint and filler to achieve a smooth surface. Use a flat hand to work the paper in a circular motion on the convex surface to achieve an even finish all round. You will need to use a former on concave surfaces.

Finishing wood

The finish you can give to wood can differ from a coat of paint to a premier finish on a fine piece of furniture. In all cases a basic minimum finish is required, the first few stages of which will be the same throughout.

If the wood is not planed — as in the case of basic construction work — you need only apply a preservative to it. Once you have planed and shaped other types of wood or joints in wood, abrasives are used to remove the irregularities. Use coarse or medium coarse grade garnet or aluminium oxide paper on hardwood.

Damp wood will not clean up to a good finish with any abrasive paper. Damp abrasive paper is useless, so always store it in a dry place. A cabinet scraper will give a fine finish on most surfaces and it may avoid the use of glasspaper altogether; use a ‘mirror’ shape former on irregular surfaces. Always work with the grain of wood, however difficult this may prove. If the paper gets clogged you may be able to clear it by running the back of the sheet over the edge of a bench – this will also make the paper more flexible and less likely to tear.

Once you have worked through the grades of paper, you can use flour glasspaper to give an overall smooth finish. To do this you need to raise the wood fibres by dampening the surface with a sponge and sanding as the wood begins to dry out. Repeat this process until a smooth, almost shiny surface has been achieved.

Avoid buying poor quality wood if you intend to give it a premier finish; sticky resin on the surface of pine, for example, will hinder finishing and may break through again after the timber is finished. If you do use such pine, scrape the resin off with a cabinet scraper.

Power finishing

This is simply the process of hand finishing speeded up. Use hand finishing when a slow approach is needed, such as on fine furniture, and power finishing when working by hand would consume needless time and effort.

Power tools are run off the domestic electricity supply but should never be run off a lighting circuit. Most power tools are double insulated, which makes them safe in all conditions except where it is very damp.

Warning

Ordinary glasspaper is not suitable for power sanding because the bonding agent used is not strong enough to withstand the harsh sanding action; close-grained glasspaper will soon clog and stop abrading. Don’t us cheap sanding discs.

Flexible disc sander

This power drill attachment is widely used for rough preparatory work on surfaces including wood, plastic and glass fibre. A rubber backing disc (or pad) either fits into the chuck of the drill or screws into the drill spindle when the chuck has been removed; the screw-in version is more secure and easier to handle. The disc fits into the pad by means of a centre locknut and plate; the pad allows flexibility to work around corners and other awkward areas.

This type of sander will not, however, give a very fine finish because the disc tends to dig into the surface of the work. A ball-joint disc is available which will give good finishing power; it does not dig into the work because the drill can be tilted without tilting the disc itself.

Use the disc at an angle to prevent score marks; angle the drill slightly to the right and away from the work, move it from right to left and hold the handle on the side of the drill to counteract its downward motion. Start with a coarse grit paper and work through to fine grade for finishing; a hand finish here will give a very fine surface, or you can use an orbital sander with fine abrasive paper.

If you use a disc sander to remove old paintwork, the paint may melt and clog the disc. Use tungsten carbide to alleviate the worst of this problem. Alternatively use a flap wheel, a power drill attachment consisting of a large number of abrasive leaves arranged in the shape of a wheel. It will remove paint on wood or metal without clogging or scoring.

Orbital sander

This type, sometimes known as a finishing sander, is available as an integral unit or as an attachment.

The advantage of an integral unit is it is always ready for use and does not need to be set up — you may also get slightly improved performance. It consists of a motor housed inside a moulded case and fitted with a pistol-grip handle; a second handle may be screwed into the case, if you want to hold it with both hands, and the trigger can be locked into the ‘On’ position. The integral unit and the attachment both employ the same method of operation and fitting of the abrasive pad.

The sander consists of a sponge rubber pad with an abrasive sheet fitted over it. The pad revolves in circles covering an area about 5mm (Ain) in excess of the size of the pad. The high speed orbital action removes high spots from every angle and gives a fine finish. Abrasive paper is available in pad-size sheets in various grades or you can cut out standard size sheets into pad size (half or one third a standard sheet) for economy. The sheet fits over the platen (or base) and is locked in at each end of a sprocketed wheel at the front and back of the platen.

The advantage of the orbital sander is that it leaves no marks. It can be used on a wide variety of surfaces, especially on bare wood (sanding with the grain); it is practical for wood finishing, polishing and buffing (using a lambswool pad) and removing rust (with an aluminium oxide or tungsten carbide abrasive).

To clean up wood, you will have to use medium to fine glasspaper by hand. You can soak the wood to raise the grain; allow it to dry slightly before sanding. Finish with fine, then flour, glasspaper by hand. On close-grained wood you will achieve a fine finish with an orbital sander without the aid of the wetting technique or hand-sanding.

When you have two pieces of wood joined at right-angles, the orbital sander is the only device which can be used without sanding against the grain of one of the pieces; on concave surfaces use the toe’ of the sander. During sanding, wood powder will form and get forced into the abrasive paper; this is not a problem and will, in fact, help the sanding action.

The orbital sander is an invaluable tool when decorating, since it can take the hard work out of the preparation of woodwork and sanding down between paint coats. The sander can also be used to rub down filled cracks or holes in wood or plaster.

Belt sander

This self-powered unit is more expensive than the other types of sander. An electric motor drives two rollers which rotate an abrasive belt; the belt is available ready made, in various grades, and is fitted to the sander with the rollers in the slack position.

Always move the sander forwards and remember to keep a firm grip to prevent it running away. Move it from one side to another across the surface of the work, straight with the grain, at a 45 degree angle to the surface. It gives a high quality finish, but does tend to leave tiny grooves. For a fine finish you will need to use an orbital sander afterwards — or finish by hand.

The belt sander is used in semi-industrial work where a perfect finish is not too important. You can hire it for jobs in the home if you have a large area to work, such as a floor or staircase.

Drum sander An attachment for a power drill, this comprises a foam drum onto which is fixed a cloth abrasive belt. It gives the same effect as a belt sander, but at a lower cost. The application of this tool has been covered earlier in the Course.

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