Posts Tagged ‘ultra-violet light’
Most paints are mixtures of three main ingredients – a pigment, a binder and a liquid. The colour and opacity of paint are due to the presence of a pigment. This can also impart considerable protection to the other ingredients by harmlessly absorbing otherwise destructive ultra-violet light. The simplest paint is whitewash which once applied is merely a coating of pigment in this case chalk. Whitewash does not offer much protection to the surface beneath it because it does not generally contain a binder (sometimes called a film former or resin). A binder holds the pigment together and sticks it to the surface. Binders are normally solids, so to produce a paint which can be spread over an uneven surface the binder is usually broken up into small pieces and suspended in a liquid.
Paints by use.
An ideal ‘all purpose’ paint should satisfy a number of criteria – it should stick strongly to the surface it is applied to, it should cover well, it should leave a decorative and desirable finish, and should last, particularly when used outside. No one paint performs all these functions well; as a result, paints are formulated for specific uses. For example, when painting woodwork, a three-coat system is usually needed consisting of a primer (to stick to the surface beneath), an undercoat to cover well) and a top-coat (to give a pleasing, durable finish).
Primers and sealers.
These should be used on new or exposed woodwork, brickwork, metalwork, plaster and so on. They will slick firmly to the surface, seal, and provide a key for subsequent coats of paint. They should also be used when you want to paint over sound old paintwork with a new. completely different, type of paint. In this situation, primers and sealers prevent chemical attack between the different paints, reduce the likelihood of the new paint not sticking to the old one and should stop the colour of the old paint bleeding through.
A primer by itself is not permanent protection for the surface below – it should be painted over with an undercoat or topcoat as quickly as possible.
Normally, you use one of these immediately after a primer when building up a paint system, or on old paintwork when you are changing the colour significantly. Undercoats are designed to have: good opacity or hiding power so that you can cover a dark colour with a lighter one without having to apply many coats of paint; a high build capability so that you can put on a thick coat of paint around corners and over sharp edges where paints tend to be spread too thinly; and a soft finish which can be rubbed smooth easily with abrasive paper (ready to lake the next coat).
These are dual-purpose paints providing both a decorative and protective final coat. They are often available in a choice of finishes matte, eggshell, satin and full gloss. The choice of the topcoat affects the overall appearance of the paint system, its durability and its ability to withstand knocks.
Some modern paints, including microporous paints and preservative wood-stains. are designed to act as their own primer on bare woodwork and, often, only one, or at the most two coats are needed.