Product Finishing

  • Created by: Zaynhamid
  • Created on: 20-05-19 12:15

Finishing Timber

Timbers are sanded before a finish is applied. Glass paper, garnet paper and wire wool are used to prepare the surface for painting, oiling or varnishing. Sanding sealer is a solvent based product that seals the surface and raises the wood fibres so they can be sanded. It is often used before applying a varnish or wax.

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Varnishes and Painting

Polyurethane varnish and laquers give an attractive and hardwearing finish to woods. They provide a tough sruface finish that can be both heat-and-water-proof and will stand up to hard knocks. They can be applied with a brush or a spray. Metals are sometimes lacquered to protect the shiny surface finish. 

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Paint and Varnish

Paint and varnish can provide colour and protection and are used on both wood and metal. Acrylic paints are water-based and are genrally not waterproof. Oil-based paints are more expensive but are waterproof and tougher. Polyurethane paint is particularly hardwearing. Softwoods tend to be painted whereas hardwoods cover less well and look more interesting when left naturally. Several different types of paint are avaliable from flat non-shine (matt) through satin to very high gloss.

Non-toxic paints and varnishes are available for children's toys and furniture. A painted or varnished finish sits on the surface of a material and requires maintenance as the coating can peel or flake off.

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Waxlpolish and sealants

When a natural appearance on wood is required, one method is to apply oil to the wood surface. This highlights the wood's own colour and grain, making it water-resistant with a non-shine finish. There are many types of oil but commonly used are:

  • olive oil - used as a finish when the wood is going to come into contact with food (for example, salad servers)
  • Danish or linseed oil - can be used on most woods; it needs two to four coats for the best production. 
  • teak oil - this is ideal for such woods as teak and iroko; it is based on linseed oil with additives such as silicone to give a harder wearing surface.

Beeswax polish is often used in interior wooden furniture to achieve a natural-looking finish. Wax finishing will not stand up to heat and many liquids may stain or mark the final surface. Specialist hard wax oils can be used on floors; the benefit of those over varnish is that the wax can be reapplied without the need for re-sanding and preparing the wood.

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Preservatives

The effectiveness of any preservative depends on the penetration. Brushing or spraying is not recommended because it only gives superficial surface protection to the timber. These methods are, however, useful where timber needs to be treated in situ and can extend the life of a product by several years if applied regularly. The internal structure of the wood can be impregnated with a preservative, by means of pressure treatment. 

Methods of preserving timber include: 

  • creosote (tar oil) - this is highly water repellent which gives it excellent weathering characteristics. Creosote-treated wood usually has an odour, but creosote is an excellent preservative for applications such as bridges, telephone poles, ailway sleepers and marine piles.
  • immersion treatments - timber is fully immersed in a bath of preservative. Hot and cold bath treatment in open tanks is a more controlled method of immersion treatment; timber is immersed in the bath and the temperature is raised to about 85°C then allowed to cool, or the timber is transferred to a separate cold preservative bath.
  • pessure impregnation - wood is placed in a cylinder which is then sealed. A vacuum is applied and the cyinder flooded with preservative. The pressure is raised until the timber refuses to absorb further preservative.
  • Tanalising - this involves impregnation with Tanalith E, an environmentally friendly wood preservatve that is applied in a vacuum pressure timber impregnation plant. The chemicals become chemically fixed into the timber and cannot be removed.
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Powder coatings

The rapid growth of wood powder coating is a result of technological advances and manufacturing expertise. Finishes can have the quality of a grand piano with a smooth high gloss, or simulate solid surface granite in a smooth or textured appearance.

Powder coating on manufactured boards is more furable than traditional paint. It is impact, chip, temperature and stain resistant, so it is well suited to hot, wet conditions like gym changing rooms and kitchen and bathroom cabinets because it won't fade in sunlight or warp in humidity. It involves the use of an electrostatic charge, both positive and negative, which offers optimum adhesion in the bond and significant strength,

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Finishes for metals

Metals are also finished by painting or lacquering. The metal needs to be prepared and cleaned before a paint finish is applied. Paints can be water, solvent or oil based. Enamelling involves a drying or curing process when, after spraying, the component is baked in an oven at a temperature of 150-200°C. This process creates heat resistant parts for metal products such as stoves or radiators. Enamelling is also used for decorative jewellry where powdered glass is used to create decorative coatings (viterous enamel).

Some non-ferrous metals have an attractive surface finish but still require polishing or buffing. Polishing removes deep scartches with a rough abrasive then continues using finer abrasives. Buffing can be done by hand or with a polishing wheel; different compounds are used with different mops to achieve adesired finish. Polymers are also polished in the same way.

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Dip coating/plastic coating

Plastic coatings are genrally applied to metals using either plastisol (a liquid vinyl) or pwder (fluidised bed). Metal parts are cleaned and degreased then preheated, dipped and then heated. During the dip, heat in the parts gel the surrounding plastic material.The hotter the metal parts are and the longer the dip, the thicker the gelled coating. During the 'cure', the plastic coating fuses. A fluidised bed is a common way of coating a product in plastic and many schools and colleges have facilities to do this. Plastic powder is fluidised by gently blowing air into the bottom of the tank. Before the powder is fluidised it is about as penetrable as fine beach sand. Once the air is switched on, the bed rises about 30% and the plastic powder looks like it is boiling at the surface.

Metal is preheated to a temperature to suit the thickness of the material and dipped into the fluidised bath. The plastic melts onto the hot metal and coats the surface. After the part is withdrawn, the residual heat in the metal fuses the coating and creates a smooth surface. For this to happen, the metal must be heated to a sufficiently high temperature to be able to retain enough heat after fluidising. Otherwise the part is returned to the oven for a very short time.

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Electroplating

Electroplating involves the coating of an object with a thin layer of metal by use of electricity. The metals commonly used are gold, dilver, chromium, tin, nickel and zinc. The object to be plated is usually a different metal, but can be the same metal or a non-metal, such as a plastic part for a car. 

Electroplating usually takes place in a tank of solution containing the mtal to be deposited on the work. This metal is dissolved from called ions. An ion is an atom that has lost or gained one or more electrons and is thus electrically charged. You cannot see ions, but the solution may show a certain colour; for example, a nickel solution is typically emerald green. The object to be plated is negatively charged and attracts the positive metal ions, whcih can then coat the object to be plated and regain their lost electrons to become metal once again. 

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Galvanising

Galvanising is a process in which steel is immersed in a molten bath of zinc to provide the steel with a metallurgical bond between the zinc and the steel, creating a barrier of protection from the elements. 

The proces consists of 3 steps: surface preparation, galvanising and final inspection. Molten zinc will not react or bond properly to the steel if it is not perfectly clean. Caustic is a hot alkali solution that removes surface dirts, paints, grease and oil. It is followed by a hot sulphuric acid bath, more commonly referred to as the pickle, whcih removes mill scale and rust. The final step in the cleaning process is the flux. The flux tank contains aqeous zinc ammonium chloride. The flux is prevents further oxides from forming on the stell surface prior to galvanising. Between each step in the cleaning process, the material is rinsed in water to prevent cross-contamination. 

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