Unlike the skills of Leonardo, Michelangelo, and their ilk, the art of the coarse painter is to transfer quickly and uniformly the contents of a can of paint onto the surface intended for coating. To properly train such a painter, requires an apprenticeship of 8,000 hours of mixed practical and theoretical training. These people then become the experts on paint application.
The paint manufacturer's interest is more peripheral and selfish, requiring only that the method of application is suitable for their product. There are some basic differences in the different methods of application that are a source of many enquiries and this memo will attempt to deal with these.
Brushing uses lateral shear forces to spread paint over the surface. This sideways movement is very important because it is an action that can displace surface contamination just prior to the wetting action of the paint. It is also an action that brings the paint into the closest contact with the surface, and for this reason it is often recommended for the application of primers. It is felt that brushing well into the surface can overcome deficiencies or holidays in surface preparation.
Whilst there are of course specific brushes for specific jobs, the best brushes invariably contain a lot of long bristles. Bristles have an excellent natural taper, as well as a flagged tip - excessive wear removes this tip and significantly reduces the efficiency of the brush.
Rolling uses a compressive force perpendicular to the surface to press the paint onto it, followed by a tearing action as the roller comes away. This action can simply roll over surface contamination whilst the tearing action causes roller marking, paint fly-off, and bubbling. High shear is applied to the paint and good wetting of clean surfaces is achieved. The pressing action is very good for coating pitted surfaces like concrete blocks. Far faster spreading rates are achieved by roller than by brush.
There are several different coverings used for rollers as well as differing pile lengths of the same material. Whilst a good background knowledge of where to use has been built up by the roller manufacturer, it is worth noting that there are several new developments taking place within the paint industry centred around new thickening systems. These have a direct bearing on application properties (such as flow, leveling, spattering, foaming) and existing knowledge regarding suitable roller types may not always apply.
Pad brushes use technology from both brushing and rolling. Whilst still relatively new in this country and the victim of reaction, it is the author's view that these pad brushes do some jobs superbly well combining excellent appearance with good speed and tidiness. Some greater difficulties in cleaning appears to be the only drawback.
Spray painting deposits a fine mist of aerosol paint onto the surface. The aerosol is produced by breaking up the paint with air jets (as in air assisted spraying) or by using jets of highly pressurised paint itself (airless spraying). With air assisted spraying large volumes of air are used which tend to hit the surface first and then bounce back from the surface. This air bounce back slows down the oncoming paint droplets and even carries some of them away. This means that the paint hits the surface with a relatively slow velocity, and hence low energy. The air however, can assist in cleaning the surface prior to paint hitting it.
Airless spraying does not have to contend with this cushion of air, and the droplets hit the surface with much more velocity and energy, thus increasing the wetting power. Airless spraying guns can deliver very high rates to the nozzle and great expertise is required to achieve uniform films.
Spraying techniques can allow operators to lay down layers of paint wet on wet and, provided the paint has the required capabilities of sag resistance and ability to dry in thick films, this is a perfectly acceptable technique of efficiently applying paint. This two coats in one method precludes the ability to monitor number of coats by using slightly different colours for each coat, but in the hands of a reputable applicator, it ought to reduce labour costs without reducing quality.
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.