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All About Paint Coatings

Understanding Paint Coatings

There are many different types of paint that are used for different purposes. Paint needs to stick to the surface underneath, give a hard wearing outer surface and look good.

Here we look at the different paint coatings used for painting wood, steel, walls, floors, plastics, etc. and the different qualities we are looking for to decide which to use.

A Paint coating has to do three things

These days paint represents a technically advanced material designed to protect and improve the look of many of the things that surround us.

First we need to understand that a coat of paint has three fundamental components each carrying out a different purpose.

  • The first and probably the most important component is the inner surface whose function is to adhere to the substrate (the surface) the paint is applied to.
  • The second is the body of the paint that has to give the paint film strength and an ability to smooth out brush strokes and hide imperfections in the surface underneath.
  • The third component is the final outer surface which most look good, carry the colour but also provide a tough skin to withstand scratching and abrasions. These days this third “top coat” may have a gloss (shiny), silk (semi matt) and matt (locally known as doff) finish.

Oil based enamel paint comes as a 3 part system

As you can imagine trying to devise a single material that can satisfy all three of these requirements is not easy. To address this issue many paints, most notably traditional oil based enamel paint used for windows, doors and frames, come as a “system” and it is usual to use three very different but complimentary types of paint applied as separate coats. The first coat is a primer to key into the substrate, the second is an undercoat, a none shiny paint which carries some of the final colour and gives body to the paint (this is often applied in two coats with sandpapering in between) and the third is a final gloss topcoat to give that shine, colour and hard surface.

Modern paints are water based, single coat paints

Improvements in paint design means that modern day paints tend to come as single layer, water based rather than oil based paints which makes the painting process and cleaning up afterwards a lot easier. They also dry a lot quicker.

Surface Preparation

As many painters will tell you the most important quality of decorative paintwork is its adhesion to old layers of paint. Most painters will also tell you that, as long as you use a good paint, the quality of a paint finish is determined by how much effort is put into preparing the original surface before painting starts.

Painting Wood

Different materials may require very different surface preparation. For example wood is a relatively soft material that expands and contracts, it absorbs moisture and it has a grain. The primer is designed to soak into the wood and lock into the grain. The undercoat will need to fill the grain and make the surface smooth. The topcoat must be abrasion resistant but must also bend and flex with the wood as it expands and shrinks.

Painting Steel

Steel on the other hand is hard and smooth. The important thing is to get good adhesion and a hard finish. Preparation of steel requires the removal of any rust and very fine scratching of the surface to allow the paint to key. Steel should always be primed with a primer that preferably has zinc in it. The zinc provides a level of cathodic protection and resists corrosion of the steel. Lead used to be used in steel primers to give this “cathodic” protection but this of course has been banned due to toxicity of the lead. If paint is soft it will be relatively easily scratched off steel and this is why paint is often baked onto metallic surfaces such as car bodies.

Painting Plastics

Plastics vary considerably according to the type of their polymer make up and paints adhere best if there is some chemical reaction between the paint and the plastic. In the case of fibre glass the paint is actually built into the final (gel) coat of the glass finish. Some plastics such as polyethylene are very difficult to paint.

Painting Aluminium

Amongst the more difficult things to paint is aluminium, a relatively soft metal which has a layer of oxide on the surface. In order to paint aluminium the surface needs to be painted with a special “etch” primer designed to eat into the surface of the aluminium and allow other paints to be applied on top.

Paints for house walls

For housing purposes there are generally three broad categories of paint in common use. The first are wall paints which are usually water-based and come in a variety of types from early whitewashes to modern day high-tech polymer-based paints. All wall paints tend to be very easy to apply and have good ability to cover rough surfaces but generally do not have the added adhesive strength or surface resistance that we require from paints used for finishing woodwork.

Taro or glue and earth paint

An unusual wall paint on external walls called Taro. Taro is made by mixing earth with glue and its grainy finish has excellent ability to hide surface irregularities. It also gives a natural look. It is very widely used and comes in many colours but is most often seen in browns where it gives a mud brick look or red to give that fired red brick colour.

Acrylic paints for woodwork

The second commonly used paint for house finishing is for painting woodwork. Until recently these paints tended to be oil based enamel paints. In recent years water-based acrylic paints have been developed for painting woodwork however, while they are very easy to use, these paints are not able to achieve the high gloss high-strength finish that enamel paints can provide. It is also important to understand that water based acrylic paints do not stick very well when applied on top of earlier coats of oil based paints.

Enamel paint usually comes in a three part form while the water based acrylics use the same paint for the undercoat and topcoat. Acrylic paints also require minimal preparation when repainted several years later. Unfortunately enamel primer and undercoat paint is very difficult to find in some countries.

Varnish for woodwork

The third commonly used type of paint are varnishes for coating woodwork. Varnishes are usually oil based and are usually transparent to allow the beautiful appearance of the wood grain to be visible. Varnishes often have a stain incorporated to darken or colour the appearance of the wood. Doff (matt) varnishes can be very useful if you want to bring out the natural look of wood.

By their nature varnishes cannot have primers or undercoats. Experienced painters know how to vary the mixture of varnish and thinning agents for each coat so that first coats can soak into the wood and top coats can be gloss or doff (matt). Varnishes can only be expected to have a fairly short lifespan if they are exposed to the sun or to weather and you should expect to repaint every two years.

Hardwearing polyurethene paints for floors

For very hard wearing surfaces such as floors and table tops a modern ”two pack” polyurethane varnish will provide a very hard surface which will last a long time but beware when exposed to the sun polyurethane has a tendency to become translucent with a “milky” look. Repainting polyurethane requires a lot of surface preparation to make sure the surface is rough to allow further coats to stick. It is also not a good idea to use hard polyurethane on a softer material such as a softwood.

Copyright © Phil Wilson 2009
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