Architectural coatings for metalwork serve two different basic functions, aesthetic and defensive (and often both). They are created to form a decorative layer over anything from the cladding on industrial buildings to the metal legs of a desk in an office. Many architectural coatings are also designed to protect metal surfaces from corrosion. Accordingly, they are used to cover all sorts of structures ranging from entrance canopies to girders.
Essentially, paints are liquids that become a solid coating once the solvent has evaporated and they can be applied onsite. This process usually takes place along with oxidation or other types of chemical reactions. Powder coating on the other hand is heated in extremely hot ovens to cure it and can only be achieved in factories.
Nearly all types of coatings available for use onsite consist of four constituents:
PIGMENT is comprised of tiny solid particles that hide the surface underneath. This layer of colour is primarily decorative, but can also be resistant to corrosion and is consequently a component of primers as well as paints.
SOLVENT is the liquid that the pigment and other substances are dissolved in to in order to help the paint flow. All architectural coatings have a rating according to the proportion of solids they contain, but the actual percentage varies within the low, medium and high categories. Typically, low consists of 20-30% solids, medium contains 30%-50% and high is up to 100% solid. Sometimes, chemical solvents are not actually intended for dissolving, instead they thin the paint down to make it less viscous such as white spirit which is mixed with oil paints. This is especially important for spray applying paint which is the most effective technique for achieving an even layer.
ADDITIVES are devised to make spray application or the manufacturing of coatings easier. They also enhance the properties of the coating itself such as preventing mould or the addition of silicone to gloss paint in order to protect metal against weathering.
BINDERS literally bind all the constituents together before they are spray applied on to the architectural metalwork. Their role is also to produce a protective film over the surface. Different types of binders include oils (either natural or alkyd), resins, proteins, vinyl, chlorinated rubber and other polymers. Binders vary chemically according to the properties called for, for example some are more appropriate for rough and dirty surfaces.
Up to four different systems of architectural coatings are used onsite, on metalwork namely primers, build coats, undercoats and topcoats. It is worth noting that other products may be used prior to these systems. Dents and scratches may be filled to flatten the surface and mordant solutions or ‘T wash’ is often put on to prepare untreated galvanised metal to degrease it before priming.
PRIMERS protect the substrate from corroding away and they also key the surface, so the subsequent coating adheres to it. These are known as ‘etch primers’. The most common primers are often labelled according to their metallic content such as aluminium, iron oxide, red lead and zinc phosphate.
BUILD COATS add thickness and also protect the metal since they contain additives for this purpose.
UNDERCOATS essentially provide a base colour underneath the finishing coat, so the top coat colour isn’t altered.
TOPCOATS are the final layers of coating on architectural metalwork. Though they can be protective, their primary function is aesthetic and they are defined as either a two-pack or a single-pack paint.
Once applied, the paint starts to dry which means the solvent evaporates away leaving the binder and pigment behind. A chemical reaction then takes place whereby small molecules link up to form long molecular chains. This polymerisation occurs either through contact with the air and is known as oxidation or by the addition of another substance mixed in just before the paint is applied. The latter is termed a two-pack coating and paint that dries merely through oxidation is known as a single-pack coating. The paints that are primarily protective are often manufactured as having two-components.
Coatings used for metalwork fall into many different categories and perform in a variety of ways. Traditional oil and alkyd oil paints are widely available and fairly easy to use. Other single-pack paints such as vinyls, chlorinated rubbers and moisture-cured urethanes are appropriate for more extreme environments. However, two-pack epoxies and polyurethane coatings last the longest and require the least amount of maintenance.
Architectural coatings for metalwork can also be formulated for very specific uses. Intumescent paints expand when they are heated and insulate the underlying metal which in turn delays the breakdown of the substrate in a fire. It is important to apply the most appropriate coating to architectural metal and a good onsite paint spraying company will help with this. Once coated properly, metal should stay in good condition for many years which saves the expense of replacement and reduces the impact on the environment.