Paint coatings

There is a large choice of painted coatings suitable for different environments and methods of application. Specific guidance on any particular type of coating is outside the scope of this publication, but detailed guidance can be obtained from manufacturer's catalogues

and data sheets. The guidance given by Corus concentrates on a few generic types of system suitable for various applications and environments. This information should be consulted as a source of information against which manufacturers' data can be compared.

In order to make rational decisions on the type of coating system to be used, it is necessary to understand a few basic concepts about paints and paint systems, as follows. Paints consist of three basic components:

1. The pigment — pigments are fine organic or inorganic compounds which provide colour, opacity, film cohesion and sometimes corrosion protection. For example, zinc and micaceous iron oxide (MIO) coatings provide significant levels of corrosion protection.

2. The binder — binders are usually resins or oils but can be inorganic compounds, such as soluble silicates. The binder is the film-forming component in the paint.

3. The solvent — solvents are used to dissolve the binder and to facilitate application of the paint. They are usually organic liquids or water. As the paint 'dries', the solvent evaporates into the atmosphere.

Until recently, rapid drying paints contained solvents, such as volatile organic compounds (VOCs). Recent environmental legislation has required paint applicators to control the amount of VOCs which are released. Paint manufacturers are therefore beginning to produce 'high solids' paints (i.e. low solvent) paint and water-based paints which do not release harmful compounds.

As the solvent evaporates, the thickness of the film reduces. The thickness of the paint coating may therefore be referred to by both its 'wet' and 'dry' film thickness. The wet film thickness can be measured by a comb-gauge allowing immediate correction of the film thickness. The dry film thickness can be measured by the use of various types of magnetic or electro-magnetic gauge. Paint thicknesses given in specifications are usually the final required dry film thicknesses.

Paint classification — the most common method of classification of paints is by their pigmentation or their binder type. Zinc phosphate primer, for example, may be used with an alkyd, epoxy or acrylated rubber binder. Care should therefore be exercised when specifying the type of paint, as the performance of zinc phosphate epoxy will be significantly different to that of a zinc phosphate alkyd or zinc phosphate acrylated rubber.

Paint systems — a full painting system will often include a primer, an intermediate coat and a finishing coat. The cost of application of the paint can be a significant factor and the tendency is to use as few coats as possible. In some cases, the same paint can be used for all three coats and the intermediate coat and finishing coat applied as one high build (HB) coat. In all cases, the various coats of paint must be compatible with each other, and the manufacturer should be consulted in order not to invalidate warranties.

The following additional points should be considered in the choice of paint system:

• The paint system should be compatible with the available method of surface preparation and the method of application. For example, site-applied systems should not generally require that the surface is grit-blasted, or that the coating is applied by spray.

• The environmental impact of the coating system should be considered. Paint systems which were once very popular, such as chlorinated rubbers, are being phased out due to environmental considerations during manufacture. Paints with low levels of VOCs are increasingly popular.

• Where paint systems are site-applied, the safety of the operatives must be considered. Expensive mobile working platforms (MWPs) or scaffolding may have to be used. Generally, brush application is preferred on site, as sprayed coatings usually have to be applied in a contained environment and the operative may require air-fed respiratory equipment.

Two pack epoxies have poor resistance to ultraviolet radiation and are highly susceptible to 'chalking'. Over-coating problems can occur when epoxies are applied in two coats, unless they are over-coated before the first coat is fully cured. This is particularly important where a two coat system is partly applied in the shop and partly on site. A 'travel coat' may be required as an intermediate coat between the two epoxy layers. Re-coatable epoxies are becoming more common.

Prefabrication primers generally have to be applied in the workshop within four hours of blast cleaning. They are usually applied in thin films, in the order of 25 microns (10 m) thick, and their durability is limited. Many modern primers based on synthetic resins are not compatible with manually prepared surfaces as they have a low tolerance to rust and mill-scale. Conversely, many oil- and alkyd-based primers, which are tolerant to hand-prepared surfaces, cannot be over-coated with finishing coats that contain strong solvents, such as acrylated rubbers, epoxies and bituminous coatings.

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