7.2.3. Plural component spraying (Two component spraying)

Plural component spraying is a method in paint application in which two-component, catalysed paints are mixed and proportioned automatically by the spray equipment system. They are used to spray coatings with high volume content solids and solvent free coatings with a very short pot life, from 30 seconds to some minutes.

The base and the catalyst are mixed at the spray gun or at a manifold preceding the spray gun. The mixed two component are then immediately sprayed onto the substrate. Each component is placed in a separate pot and then drawn through lines to the manifold or spray gun. In this way spraying can continue with out frequent stopping and flushing the equipment with solvent.

7.2.4. Float coating

This method has been used to apply non-drying or semi-drying oils or derivatives. The method varied somewhat from manufacturer to manufacturer, but the principle is that the product is poured on to the water surface in a tank. The coating is transferred to the tank walls by raising or lowering the water level. Tank tops and bottoms are difficult to protect by means of this method, but such problems can be reduced by combining it with spraying. The danger of contaminated waste water and the occasionally doubtful efficiency of this method mean that it is seldom used nowadays.

7.2.5. Paint-drum pump

As an alternative to high-pressure spraying, soft coatings may be applied using a simple low-pressure paint-drum pump.

7.3. Treatment of equipment before and after application

Cleanliness is definitely the most important factor in the correct handling of airless spray equipment. In fact, most problems we meet in practice are due to faulty or poor cleaning of the equipment.

7.3.1. Before application

During storage or transportation or actually at the site a lot of impurities can be collected on the lid of the can. When starting paint work, the first thing to be done is to thoroughly clean the lid of the paint to prevent any of these impurities from entering the can when it is opened. When stirring the paint, a mechanical agitator is highly recommended. Never make use of any old piece of wood lying around as a stirring implement (It will introduce dirt or splinters to the paint). When the paint is well stirred the tin may be put directly under the spray equipment. If so desired the paint may be poured into a special container attached to the spraying equipment. However, it is of vital importance that this container is thoroughly cleaned beforehand.

Some types of spray equipment are fitted with a special paint intake feed which can be lowered directly into the original paint can. The reduction valve is set at a pressure of 1-2 kg/cm². At this point no nozzle is attached to the spray-gun. The trigger on the spray-gun is pressed and the thinner in the equipment is pumped out. This thinner may be kept for later washing. When the paint has forced all the thinner out of the system the trigger is released. The spray-gun is subsequently pointed into the paint container, the trigger is again pressed and the paint allowed to circulate through the system to remove any residual solvent present in the equipment.

The size of the nozzle must match the viscosity of the paint (see data sheet for recommended tip size). In general, high viscosity materials require larger orifices to assure adequate flow. Low viscosity materials require smaller orifices.

Before the spraying work begins it is important to perform adjustments of the system pressure. This is done according to the following procedure: Point the gun at a target. Then increase the pressure until the proper spray pattern is achieved. Do not use higher pressure than necessary for achieving a proper spray pattern. Too high pressure will increase the formation of dust (dry spray). If a proper spray pattern is not achieved then the first thing to do is to try another nozzle. The second alternative is thinning of the paint. However, changing the viscosity of the paint by adding a solvent or by heating must be in conformance with the requirements of the paint manufacturer and as expressed in the technical data sheet.

Another reason for having a bad spray pattern is that too small dimension of the material hose is used. With heavy duty paints hoses with dimensions of 3/8” should be used. Over thinning must be avoided because you may not be able to achieve proper film build.

Remember, the nozzle is the most important part of the airless spray equipment. Always choose the correct nozzle for the job. The nozzle is very expensive and should be handled with great care. Never use a metallic object for cleaning the holes of a clogged nozzle. If necessary, wooden splinters may be used but never any harder material. After cleaning, the nozzle is fitted and the equipment is ready for use. By using reversible nozzles you can in most cases overcome the problem with clogged nozzles.