FINISHING METAL

You may be satisfied with the condition of the metal you are working and be prepared to keep its natural finish. However there are many ways of enhancing the metal which will also provide protection against corrosion; this is particularly so in the case of ferrous metals. The effectiveness of any finish will, of course, depend on the conditions to which the finished article is subjected.

PREPARING SURFACE

The most common method of finishing metal is painting it’. But. as with any finishing process, preparation is most important since no finish you apply will cover up blemishes in the metal. These must be removed before any final work is tackled.

USING ABRASIVES The initial preparation of the surface can be done by filing. Start with a coarse file and graduate to a fine one. draw filing across the work. Then use emery cloth; again the principle is to start with a coarse grade, continue with a medium grade and finish off with a fine grade. Wrap the cloth round a wood block – or use an abrasive block – and work backwards and forwards in a straight line, if possible parallel to the long edge of the work. The final rubbing down with fine grade should be done with a lubricant; any mineral oil. such as motor oil, will be suitable for this. The secret of a good finish is basically hard work; the more time and effort you are prepared to spend rubbing down, the better the final surface will be.

A disc sander attached to an electric drill will take the hard work out of rubbing down the metal: but bear in mind this drastic treatment could alter the shape of the metal if not applied carefully. DEGREASING Any grease on the metal will affect the final finish, so even the smallest traces must be removed. This is best done by swabbing the work with white spirit. Apply the spirit with a clean lint-free rag; and keep your hands off the work since this can affect the final result. Either hold the work in a vice or wear cotton gloves.

When working with steel you candegrease the metal by washing it in a caustic soda solution. This should be made up of 227g caustic soda and 7g washing-up liquid or detergent to 4.51it water; the detergent helps to hold the solution on the metal. Heat the solution to just below simmering point, about 90 C, in an old pan on top of the cooker.

For aluminium you can degrease the metal in A solution of 57-85g of washing soda to 4.51U water, heating it to the same temperature as for steel. It is also possible to wash the work in a bath of dilute hydrochloric acid heated to about 40°C.

In all cases the work should be thoroughly rinsed in hot water after degreasing. Warning Chemicals – and in particular acids – can be dangerous if not handled properly and certain precautions must always be taken. When handling, always wear acid-resistant, heavy duty gloves, and never let acid touch your skin. If it does, wash it off thoroughly for several minutes in cold water and then get medical advice. NEVER add water to an acid; always add acid to water. Keep the liquid in a scaled glass container specifically made for storing acids; any container must be clearly labelled. When pouring, use the side away from the label to prevent any drips destroying the label. Always store in a locked cupboard used solely for that purpose so there is no danger of unsuspecting hands touching the acid or chemical. Finally, never use the containers for any other purpose.

The work you can do on metal which involves chemicals will be dictated by the availability of the chemical concerned. The only outlet for the DIY worker is probably the local chemist. Measuring temperature Because of the difficulty of gauging temperatures, particularly when dealing with acid and other caustic solutions, it is well worth investing in a laboratory thermometer, which will measure up to 105°C.

Polishing metal

For a fine finish, you can use polishing mops. These are made of calico, covered usually with lamb’s wool, mounted on a spindle and are used fitted to an electric drill. When used with a polishing compound, the finishing process is known as buffing.

Buffing

This provides a bright metallic finish and is commonly used when working non-ferrous metals such as aluminium, copper, brass, pewter and silver. First remove all scratches with emery cloth as already described. Rub the work with a pumice stone to remove the smallest scratches that will be left. The compound, which comes in the form of a stick, should be rubbed against a spinning mop so it is deposited evenly over the calico surface.

Fix your electric drill in a special stand secured to a workbench or rigid surface and apply the work to the mop. Always wear strong gloves in case your hand slips while polishing. Work below the centre of the mop, drawing the metal towards you in the opposite direction to the rotation of the mop; if the work slips or is snatched from you, your hands will not be pulled against the wheel. To achieve a final high polish, carefully draw the work lightly into the mop, again below the centre.

A reasonable finish can also be obtained by applying a proprietary metal polish to the work, although this will involve a lot of hard rubbing. Crocus powder, a fine polishing powder, is also suitable for finishing metal. This can be applied with a soft polishing mop, as described above, or by hand with a damp lint-free rag.

Painting metal

A painted finish involves applying a primer, undercoat and then one or more layers of top coat, depending on the use to which your metal will be subjected and the conditions under which it will be kept. It is important the metal is completely free from rust before any paint is applied. You can use an ordinary proprietary rust-killer or a combined rust remover/preventer which leaves a protective film on the surface. There are also proprietary zinc and lead undercoatings which will give extra protection to the metal before painting.

The thickest paint application will be obtained with a brush, although brush marks will be hard to avoid until you have mastered the technique. It is easier to apply the primer and final paint finish with an aerosol can or spray gun. A whole range of matt, cellulose and metallic finishes is available in aerosol cans so you can get exactly the effect you want. Before applying a spray-on paint, always practise on a piece of scrap metal.

Colouring metal

Other finishes on pieces of metal can also be achieved by colouring. For example, a blue/black finish can be given to ferrous metals by heating the metal until it turns the desired colour and then dipping it in a mineral oil; this is only practical with smaller items. With larger pieces of work you will have to coat them with oil and then apply heat to all areas with a blowtorch.

Alternatively you can rub heated metal with woollen cloth. The effect this has is to scorch the wool, which is deposited on the metal. The work should then be rubbed with oil.

To produce green colouration on brass and copper, brush daily with a mixture of vinegar. salt and sugar. Use enough vinegar to dissolve one teaspoon of salt and one tablespoon of sugar. After a few days, the metal should change colour.

To get a matt finish on aluminium, degrease the metal and dip it for a few seconds in a solution of 227g caustic soda to 4.51it water heated to about 80°C.

WARNING

With any colouring process always experiment on a piece of correctly prepared scrap metal to check on the desired finish before working on the final METAL.

PLATING METAL

Plating is another form of metal finish which gives extra protection. Methods used on iron and steel include hot-dip galvanizing, where the work is dipped in molten zinc. This process is used on such things as buckets, guttering and corrugated iron.

Tin plating is carried out by the hot-dip method as well and is used for containers and light domestic items such as baking tins.

Zinc plating gives a higher degree of protection against corrosion than tin because the tin merely protects the iron or steel from the atmosphere while the zinc protects sacrificially; that is the zinc corrodes while the basic metal is preserved. Small items are coated with zinc by rotating them in a heated drum containing zinc dust, which produces a matt grey protective covering. This process is known as sherardizing.

Lacquering METAL

With softer metals such as copper and brass, a high finish is fairly easily obtained by the processes already described; but exposure to the atmosphere will inevitably result in oxidation. Ordinary varnishes, such as those used for wood, will not normally be sufficient, since once oxidation starts it will break through the varnish coating and the work will start deteriorating. A great deal of effort will then be involved in cleaning off the ‘protection’ before you can refinish the metal.

There are special metal-protection lacquers -often used by restorers of antiques – which give an impervious coating that should last for quite a few years. Again care must be taken to ensure the work is not touched by hand between the final degreasing and the application of the lacquer. Cold lacquering is a difficult process; hot lacquering is easier and will prove just as effective.

Wash the work in methylated spirit and place it on a thin sheet of iron or an asbestos mat on top of a gas ring so it is heated gently – a camping gas stove is ideal for heating or you can use a blowtorch. The work should be turned frequently to ensure even heating. Apply a thin coat of lacquer with an old paint brush, but take care to avoid any brush marks. If the work is small, you will be able to dip it into lacquer, which must first be warmed: this can be done by placing a suitable vessel containing lacquer into a pan of boiling water. Allow the work to cool by removing the source of heat.

A colourless lacquer can be made from methylated spirit and shellac by mixing 25 parts spirit to one part shellac. Lightly brush this lacquer onto the work, place the work on a large metal sheet and heat it over a low flame to avoid the lacquer catching fire.

BURNISHING METAL

Soft metals can also be burnished, a process which is really an extension of polishing. The surface of the metal is rubbed hard with a steel or agate rubber in whatever shape suits the work. Agate is not readily available to the DIY worker: but you can instead use a small piece of high quality steel such as silver steel. The effect of rubbing this on the surface of your work is to alter the grain of the metal and flatten it as you remove all surface marks. Soapy water should be applied as a lubricant.

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