1.5.2 Finishes for polymers, woods and metals

Finishes for metals

Cellulose and acrylic paints

Paints provide colour and sometimes a textured finish to metals.

They can also provide a barrier against corrosion for metals such as Steel and Iron.

The surface of the metal must be cleaned, de-greased and primed for the paint to adhere to the surface properly.

To get a full, rich colour an undercoat is often applied before the colour and, in the case of cars and other shiny products, a clear lacquer may be applied over the paint layer.

Electroplating

This involves using a metal to coat a (usually) cheaper metal to provide a protective layer and enhance the aesthetic qualities of the material.

For example, teapots can be silver plated and cheaper jewellery can be gold plated.

The metal product is submerged in a solution containing tiny particles of the plateing metal. When the current is applied, the product attracts the donor metal and the product is electroplated.

Polymer dip coating

This is used on a variety of products such as wire coat hangars and outdoor equipment.

The metal is heated then the hot product is dipped into a tank of fine polymer powder which has air blown into it to ensure even distribution of the powder.

The hot metal melts the powder giving a tight fitting and even polymer finish.

Metal dip coating

Metal can also be dipped into other metals for coating.

In this case the product is dipped into a tank of molten plating metal. This process is often used for inexpensive metals such as low carbon Steels primarily to provide a barrier against corrosion.

Food cans are often Tin plated because it is less reactive to food goods

*Galvanizing

Using a Zinc plating is known as Galvanizing. It is used to produce car bodies, buckets, screws, hinges and much more. It give a corrosion resistant finish

Powder coating

This process gives a much better finish than dip coating.

A negatively charged product is sprayed with positively charged polymer powder so it sticks to it like a fine dust.

It then passes through an oven which bakes the fine layer of powder into an even coat of polymer. Used to coat chair and table legs as well as many domestic metal products such as fridges and washing machines. *We saw this at triumph motorbikes

Varnishing

Metal varnishing is a method used to provide a clear surface finish to metals that will also provide some resistance to corrosion.

They would be applied exactly the same as a paint or spray. Varnish is effectively paint without any pigment leaving it completely clear. A clear lacquer could also be used for a finer finish.

Sealants

Metal sealants are tough polymer based sealant coatings that protect polished surfaces from decay and tarnishing.

Silicon based sealants are often used in the automotive industries as an additional bodywork barrier against weathering and rust. It is generally applied with a cloth or machine pad to produce a film that is then left to cure for 15 mins then buffed to a shine.

Preservatives

Metal preservatives can provide temporary in-process or final post-processing protection for metal surfaces. They are ideal for moulds and dies to prevent fingerprints showing on surfaces. They can be applied with a cloth, spray or via immersion.

anodising

This is a finishing process commonly used for Aluminium products to enhance the natural oxide layer and provides increased hardness and toughness.

It is similar to electroplating but penetrates the surface of the material which can then be finished with a lacquer or varnish to further protect the finish.

Cathodic protection

All metals have a natural voltage and when two different metals are joined together there is an electrical current. If water is present at the joint it results in the formation of an electrochemical cell and one of the metals will corrode.

Cathodic protection is a method used to control the rate of corrosion. The main use is to protect steel structures buried in soil or immersed in water such as pipelines, ships and offshore platforms.

The technique of providing cathodic protection to steel preserves the metal by providing a highly active metal that can act as an anode and provide free electrons. By introducing these free electrons, the active metal sacrifices its ions and keeps the less active steel from corroding.

finishes for woods

Wood is a natural product and is therefore prone to attack from moisture, insect and weathering.

Although wood is often used for its attractive grain, it can be further enhanced by the use of wood stains or colour washes to change its colour or shade. These are often combined with varnish to provide additional protect for the wood.

More importantly, finishes for wood should preserve the material. Wood can degrade in dry conditions as well as wet condition and is susceptible to wet and dry rot.

The following chart lists the main finishes for woods. How they are used depends on the function and aesthetic requirements of the product. For example, a chopping board would not be oiled or waxed as it would contaminate the food. A cricket bat is oiled as a varnish would chip away in use.

summary

There are three terms associated with finishes.
Materials may be self coloured.
Materials may be self finishing.
Often these types of material will not require a surface finish applied but they may be applied for aesthetic reasons.
Materials may have a surface finish applied or they can be left unfinished.
Finishes can be applied, plating, coatings, preservatives or a combination of these.
There are to main reasons for finishing a material: to protect (preserve) or to enhance (aesthetic).

Revision Notes

For each material group, you need to be familiar with the range of finishes available and that are suitable.
A range of printing processes are available and are used depending on the finish required and/or the scale of production.
Materials can be self-finishing/coloured or they may have an applied finish.
Many polymers are self finishing/coloured
Ferrous metals will require a finish to protect from decay
Non-ferrous metals wont necessarily need a finish
Wood finishes can enhance the look of the wood and provide protection from weathering, rot and insect attack.