The main objective of a paint is to create a film once applied to a surface. Once applied the film then has to do three things:
- Disguise the surface below
- It has to stay glued to the surface
- It must protect the surface below
This is achieved through the three main ingredients - The pigment, the binder and the carrier
Provides the colour effects and hiding ability to the film
This has the job of binding all the particles together to form a continuous film when the paint dries and it also bonds the two surfaces together. In more traditional paints this used to be a natural substance such as glue size or linseed oil, depending on the paint type but in today's more modern paints this is usually synthetic resins such as alkyd, acrylic, vinyl or polyurethane.
This final ingredient makes the paint flow very smoothly as it is applied to the surface. As the paint dries this substance evaporates.
The main domestic paint types have different carriers. The Water-based paint types have the pigment and binder suspended in water as very tiny droplets. This tends to be a milk like, emulsion and is aptly named "Emulsion" (In the USA this is usually referred to as Latex paint). As the water begins to evaporate the droplets coalesce and form the film of the paint.
The Solvent-based alkyd paints have the binder and pigment dissolved in a petroleum based solvent. These types of paint tend to take a lot longer to dry than the water-based paints (In the USA these are know as oil, or oil-based paints). A characteristic of this paint type is the "painty" smell as it's drying. many people find this rather unpleasant. With the great awareness at the moment concerning the growing health risks associated with the inhalation of solvents, these paint types are becoming rather un-popular.
Thixotropic, also know as non-drip, contains a range of additives to increase its performance. In this case the additives go towards allowing the user to load more paint onto a brush and a thicker paint film can be applied. One coat of this type can often be sufficient.
The application of one coat in most cases is never enough and a paint system must be applied to ensure that there are plenty of coats. The nature of this system often depends on the paint being applied and the surface being covered.
The first coat will be a sealer and will be used where necessary to seal the surface. The second coat will be a primer, which will provide a good key for the paint film to adhere to.The third coat will be an undercoat, which builds up the film to form a flexible, non-absorbant base of uniform colour, often very close to the fourth and final coat, the top coat, which provides the colour and actual finish
Varnishes and Wood Stains
In essence, varnish is basically a paint without the pigment applied to it. The majority of these are polyurethane based resins and are solvent based (similar to oil paints). Acrylic varnishes, although water based, are becoming more popular for their environmental and health benefits.
Similar to paints, varnishes are also available with a satin/silk or high gloss finish, either clear or with the addition of a very small amount of colour. Varnishes with added colour are often intended to enhance the appearance of wood, or even give it some extra colour.
Varnish is also its own primer and undercoat, although in some cases it is better to thin the first coat with roughly 10% white spirit (for solved varnishes) and water for water-based varnishes. In most cases it is also better to apply this with a lint free cloth and you are able to rub the varnish into the wood grain better than you could do with a brush.
Once the first coat has been applied and it has dried, it then has to be keyed (rubbed down lightly) and a second coat is then applied (without dilution).
Unlike paint, varnishes and stains are designed to soak into the wood. You can even apply a clear varnish to the stain to improve the finish and also make it more durable. These are also available in solvent and water based variations and also in a variety of colours and shades.
As mentioned stains are often applied with a lint free cloth. This aids in the blending and coverage of the finish, but you will have to work quickly to blend the wet and dry edges together to avoid any hard lines and overlaps. A water based stain will also raise the fibres on the surface of the wood which in many cases also spoils the evenness of the colour.