Paint Application

Paint is not a finished product until it has been applied and dried on an appropriate substrate at the designed performance film thickness. Proper application therefore is critical to the performance of the paint system.

High performance paint systems are specially sensitive to misapplication thus knowledge of the application characteristics and recommended film thickness are vital to obtain optimum results.

Weather Conditions

Bad weather conditions are a perennial hazard in painting operations. Paint should never be applied on wet surfaces and therefore painting should be avoided not only in rain, sleet and fog but also when high humidity and low steel temperatures lead to condensation. Condensation is very difficult to detect on surfaces and will occur if the steel temperature is below atmospheric dew point.

In general , application should not take place when the steel surface temperature is less than 3º C above the dew point. Extreme temperature may present problems too. At low temperatures (below 5ºC), the curing of paints such as epoxies may slow down dramatically and for some paints stop altogether.

At high temperatures, solvent loss from paint atomized during application is very rapid; paint droplets do not coalesce on the surface (leading to a porous coat) and clouds of dry spray may also be produced. The problem may be rectified by the addition of thinners but these should never be more than a few percent. Generally, painting should be avoided during extremely hot hours – where paint operations are carried out in hot climates, the paint should be applied in the morning and early evening.

Application Methods

Three main methods are used in painting. The choice depends on the type of coating to be applied, the effect on adjacent areas and the degree of skill of the personnel.

Brush & Roller

Low viscosity paints are easily applied by these techniques to yield low applied film thickness. Modern, thixotropic paints are often specified at high film thickness specially where they perform a protective function. Therefore, where brush and roller methods are called upon (specially for "touching up "or" stripe coating") a number of coats may need to be applied in order to achieve the minimum specified dry film thickness. Although these techniques have largely been replaced by spray application, they may find use in maintenance schedules.

Conventional Spray

This technique mixes a jet of air with a stream of paint to propel a fan of paint droplets towards a surface. The mix of air with the paint particles gives high turbulence however and considerable "bounce back". Air atomization of paint can thus results in considerable over spray. Therefore not only adjacent areas must be protected but also paint losses may vary from 20% to 40% on steel and paint operatives must wear protection to avoid paint mistinhalation. The technique particularly suits low viscosity paints and is most commonly used for the application of conventional decorative paints and zinc silicate coatings.

Airless Spray

This technique relies on hydraulic pressure rather than air atomization to produce the spray. Paint under very high pressures (1000 to 6000 psi, approx. 100 to 400 kg/cm2) delivered to the spray gun and then forced through a very small orifice to atomize it. Thus there is more rapid coverage with much less over spray and much higher film thickness can be obtained. Most of the products manufactured by National Paints can be applied by airless spray. Some products (e.g. anticorrosive) are designed to be applied at high film build, others (e.g. finishing paints) at low thicknesses. Follow the recommendations in the Product Data Sheets. If the recommendations are significantly exceeded and over-application results, sags and runs may develop - these are not only unsightly but may also have detrimental effects on the performance of the coating.

Finally, it must be remembered that airless spray ejects paint under very high pressure. Do not direct the spray at people nearby as injury can be easily caused and take due precautions when the equipment is being cleaned.