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)
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.
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.
Iron and Steel - mild steel, tool steels.