Surface Finishes

Surface finishing both enhances the appearance of the materials and usually protects it from the environment ( e.g. wear, corrosion, attack by water,insects and fungi etc)

For Wood

Paint - Water based paints are used in school because they are non toxic and quick drying. Emulsion paint as used on walls at home can be used but ACRYLIC paints are tougher and more durable. A primer can be applied which will stick thoroughly to the surface and give a matt finish. Two or more topcoats of the coloured acrylic can then be applied. A foam roller gives an excellent finish, foam brushes can also be used. A light sanding between coats ensures a smooth final finish. Oil based paints can be used but are slow drying. Solvent based paints usually in aerosol cans will give a very good finish but the solvent is a fire risk and is also toxic.

These paints can be used on metals as well. The use of a primer to ensure that the surface is chemically neutral is essential when painting metals, especially steels.

Varnishes and Lacquers - these are similar to paint but they are clear/translucent. They are available in matt, satin and gloss finishes the same as paint.

Oil - teak oil used on garden furniture is the best known. Linseed oil can also be used on many types of wood (linseed oil is the basis of traditional oil paint and is also used to make putty). Danish oil is a quick drying oil which can be used on any timber to enhance its appearance. Vegetable oils can be used on wooden food utensils.

Wood stains - colour the wood but need to be protected with varnish or lacquer. Coloured varnishes and oils are also available.

Sanding sealer - usually cellulose based (solvent). Very quick drying and used to seal the surface which is then sanded to flatten the surface before subsequent coats of paint or varnish are applied. Very effective when a wax finish is to be applied. Water based versions are now available.

For Plastics

Plastics are usually polished or protected against damage to keep their manufactured finish e.g. acrylic. To polish plastics or metals it is essential "to go through the grades". This means start with a coarse grade of abrasive paper to remove deep scratches, then go onto successively finer grades removing feinter and feinter scratches. Wet and dry abrasive papers are usual. When the finest grade paper has been used a metal polish can be used to give the final highly polished effect. A buffing wheel can also be used for the final stages where a calico wheel coated with an abrasive soap removes scratches and polishes the surface.

Self finishing - many products are manufactured with their final finish. Injection moulded products are formed in a mould which is either textured or highly polished - this finish is imparted to each product produced by the mould. Most plastics are manufactured with a very good surface finish.

For Metals

Iron and Steel - mild steel, tool steels.
Steels rust (except stainless steel) so they need to be protected against moisture and air.
Coating in oil or grease is effective as long as the coating is not removed. All of the steel that is delivered to the school is greasy.

Plastic dip coating is very effective as it can change the appearance as well e.g. the shelves in fridges and dishwashers are plastic coated. In school the steel item is heated up to straw colour (blue is too hot), then dipped into a fluidising tank, where air is blown through the polythene powder to make it fluid enough for the item to be dipped in. The coated item is then removed and allowed to cool. Any excess coating is trimmed off with a knife.

Painting is the most common way of protecting steel from rusting e.g. car bodies, white goods, filing cabinets etc. The paint process is:- First to abrade the surface to make it smooth and to give it a key (rough so that the paint will grip). Second degrease using a solvent to dissolve any oil or grease. Third apply a primer which grips securely to the surface and will also grip the next layer of paint. Fourth apply finishing coats, sanding down between coats if hand brushing.

Blue and oil. This a very traditional method used on tools and springs. Heat the item up to blue then quench in oil.

Lacquer – this a clear finish like a varnish. The surface is thoroughly polished or patterned, degreased and then lacquered.

This is used to apply a layer of another metal such as chromium or nickel which do not rust and can be highly polished.

Galvanising is a form of electroplating which applies a layer of zinc a dull silver grey finish is usually produced. The layer of zinc is usually thick to protect steelwork used outside e.g. fences, water tanks etc.

Aluminium alloys.
Aluminium is very resistant to corrosion under most conditions because it forms a tough but very thin layer of aluminium oxide as soon as it is cut. Salt water will attack aluminium however so it is replaced by stainless steel and brass for maritime use. (boats and ships at sea). Aluminium can be polished or have its surface textured e.g. satin finish.
Aluminium can be painted.

Anodising aluminium is an electrical process which makes the oxide layer increase its thickness. This thicker oxide layer can be dyed any colour.

Brasses and bronzes.
These alloys are usually simply polished and lacquered or left unfinished. They are resistant to serious corrosion in normal use but do become dull.

Quiz 1 - Wood Finishing

Quiz - 2 Finishes matching

Quiz - 3 Plastics finishing

Quiz -4 Metal finishing

Quiz - 5 Finishes (Metal and Plastic)