All paints are not created equal, so simply asking for two coats of paint is like asking for a slice of biltong. Don’t be disappointed if your slice is too small, as you didn’t specify. The same applies for painting projects. Two coats of a cheap, low quality paint will not provide the same coverage or finish as one coat of high quality industrial paint, nor will it last as long. So what determines the quality and durability of a given paint?
To understand what makes up a high quality can of paint, we must first understand that there are four main raw materials used in paints, namely 1) solvents (or water), 2) pigments, 3) resins (binders), and 4) additives.
Solvents refer to various low viscosity, volatile liquids which can dissolve a resin. Solvents include petroleum mineral spirits and aromatic solvents such as benzol, alcohols, esters, ketones, and acetone. Water is used in many paints. The solvent makes up the liquid portion of the paint which makes it easier to apply, but evaporate as the coating dries. A paint with a higher volume-to-solids ratio will contain less solvent and more pigment and resin which form the coating on the wall once the solvent has evaporated and the paint is dry.
Pigments are simply insoluble, finely ground materials that give paint its colour. Titanium dioxide is the most common pigment as it is used make basic white paint.
Resins are used as a binder in paint. Resins can be translucent or transparent, solid or semi-solid. The nature and amount of binder determines much of a paint’s properties, such as washability, toughness, adhesion, elasticity and colour retention.
The last category of raw materials in paint are additives, which can serve many purposes, from reducing the drag on a brush, providing anti-slip characteristics, adding texture, etc. There are many different paint additives, such as driers, anti-settling agents, anti-skinning agents, defoamers, silica (or sand) for texture, mildewcide, perfumes and a host of others that enable paint to cover well and last long. Some, like calcium carbonate and aluminum silicate, are simply fillers that give the paint body and substance without changing its properties.
Application of a paint consisting mostly of solvents may be easy, but when the paint dries it leaves behind less of the other raw materials on the walls. This sort of paint may only have a 28-30% volume-to-solids ratio and so when the paint dries the thickness of the paint film that is left on the wall is relatively thin. If most of the remaining materials in that paint are fillers, like calcium carbonate, then your walls may look fine after a two coat application, but the coating will not last very long.
The dry film thickness of paint is measured in microns (µ), which is a thousandth of a millimeter. Many commonly used paints produce a dry film thickness of around 25-40µ per coat, so a two coat application will leave your wall with a film thickness of 50-80µ.
A high quality paint with a higher volume-to-solids ratio will produce a thicker film when it dries. Ideally, the other raw materials, such as the resins, should help to contribute toward a quality coating that will last for a number of years, or even decades.
This is why it is important to specify a minimum dry film thickness (DFT) rather than arbitrarily insisting that a contractor apply two coats of paint.
That depends on a number of considerations including the area to be painted, type and condition of the substrate and how long you would like the coating to last.
Matt and Flat Finish
A non-reflective appearance that helps hide surface blemishes, making walls look smooth. These finishes cover imperfections in surface and application and allow for easy touch-ups, although they tend to hold dirt and are difficult to clean as cleaning can actually remove the finish itself. This finish is best suited to adult bedrooms, dining rooms, formal living rooms and ceilings.
Medium Sheen and Satin Finishes
With its slight gloss, this finish offers the benefits of richer look and actually retains its pearl-like finish once dry. This finish stands up well to washing, although it does not hide imperfections as well as a matt finish and touch-ups will stand out. It works best for high-traffic areas such as hallways, woodwork, children‘s bedrooms and family rooms.
High Gloss Finishes
These paints reflect the most light once dry, providing a bright sheen. These durable finishes offer high resistance to moisture and are easy to scrub clean, although they show up every imperfection, require additional prep work and do not allow for touch-ups. This finish is perfect for areas that are cleaned frequently, such as kitchens, bathrooms, utility areas, cupboard doors and trim.
You may read more on the top by looking at the article: “NoTwo Coats Are Created Equal!“