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Airless spray coating - Maximising your investment

The principle of an 'airless spray' is simple, consisting of a high pressure pump that atomises paint at point of discharge.

The clever part is how well the various brands and designs have refined the machinery to deliver outstanding performance, capacity and reliability.

In addition to the skills of operation of airless equipment it is essential to understand the potential of 'airless paint' applications to obtain satisfactory returns on a significant investment item that also has the potential to cost you a great deal of money!

Often the true costs are disguised by spectacular reductions in time, but at actually hugely increased material costings.

Therefore careful consideration of the potentials and comprehensive planning can ensure that all contingencies are met and the best results obtained.

Question everything to decide on a battle plan. (After repeated experiences of course this becomes a natural process).

Some of the answers are simply yes or no and you can probably add to the list. It's definitely not funny when all is ready to go but there's no power available!

However the reason to plan as best as possible is when you pull the trigger, paint comes out and lots of it. If you're not prepared you can cause some major damage, injury or waste a lot of product.

The characteristics of paint need to be understood relative to being subjected to the high pressure (and volume outputs) or airless application. Apart from the mess poor operators can make (and poor planning adds to paint problems), material overruns turn problems into disasters.

Paint manufacturers usually support their product with Product Data Sheets that describe the usage of the product and recommends Spreading Rate (expressed as Area (square metres) per litre = SR and the dry film thickness which is the thickness of the material (usually one coat) after properly drying. The thickness is usually in microns expressed as um = DFT.

Other information is often volume solids of the material, which is the amount of material in the can, excluding thinners and solvent expressed as a percentage = VS.

Because selling paint is a highly competitive industry the recommendations are the manufacturer's best average estimate considering the coverage, life expectancy, appearance and ease of application. Results wildly above and below the nominated spreading and thickness rates seldom perform with any satisfaction and generally cause a great deal of grumpiness. Because we know from the Data Sheet the spreading rate per litre, we can easily determine the cost of the material per litre for estimating work packages, and most calculations in the paint industry are based in this manner.

Because airless pumps have a high output (relative to area potentials), these sort of results occur often:

Medium sized and small airless pumps typically used in the painting trade have average outputs @ 3000 psi between 2 litres and 4 litres per minute. (The smaller capacity supporting a 17-19 thou tip).

The potential in output areas:

@ 2 litres = 20 square metres per minute
@ 4 litres = 40 square metres per minute
or 1200 to 2400 square metres per hour

Therefore proper management of the process assures some urgency, particularly when industry criticisms are:

All these are a sure sign something is very wrong and indeed it is because the costs are fantastic and you are paying!

There is a simple discipline to develop and practise to return the costs of materials to industry norms and enjoy the productivity (labour) savings and make real money.

  1. Technique - application has a set of skills.
  2. Tip and Pressure Selection - there is a balance.
  3. Materials - Paint - the sole purpose of above.

Application technique

Appreciating the paint pressure produces a fan of paint only, there are no 'soft' edges of the fan spray pattern and a system of application therefore is apparent. We need a practical plan to minimise difficulties relative to overlaying heavy coatings of the paint. Operators often attempt to compensate by adopting a 'wristy' laying off motion.

The correct attitude of the spray fan is always to maintain the fan angle at right angles to the plane of work. Any deflection, either horizontally or vertically produces at one side a thinner application, but loads the other side with an overthickness, both unsightly and likely to run.

The correct technique is with the gun at right angles and always parallel to the work face with the gun travelling at the speed that the spray fan slightly undercovers the surface. The return spray, also parallel, overlays by 50% thus, stroke by stroke, attaining full attractive coverage of the surface.

This sounds complex but is actually easy to do. However, it is not easy to correct those operators that persist spraying with a loose 'wristy' action. Short of breaking their arm and applying a plaster cast, this remains a difficult habit to correct. Left handed people rarely have this problem, possibly because they are already living in a largely right handed world and are more receptive to adaptation.

It goes without saying that all operators should be understanding of the equipment, its operation and day to day care, the safety concerns to exercise and the health contingencies.

Spray tip selection and pressure

Selection of the best size and fan width are critical to the economical function of spraying. The size is in the thousandths of an inch and is designed to produce optimum results at 300mm distance from the work.

The tip size is preceded by a number (e.g. 413, 513 etc) denoting a 13 thou tip size. The 4, 5 etc refer to the angle of spray - i.e. 40 degrees, 50 degrees and roughly double the number (4 x 2 - 8) equals the width of spray @ 300mm - similarly 5 x 2 = 10 and so on, fan widths in inches.

The tip size matches the viscosity of the material and standard matches are:

Note that the spray tip nozzle diameter is always an odd number. Fan tip sizes are always even. All spray tips must be fitted with a guard (not to protect the tip but the user). The line filter size must match the tip size (this is a common fault - mismatched filters). Usually tips are quickly changed without downtimes and are reversible for self-cleaning.

Airless pumps have a fluid pressure adjustment and the ideal situation is to have selected the best tip for the material being sprayed and adjusted the pressure to properly atomise the spray flow. Because the spray tip opening is elliptical, too little pressure prevents full atomisation and a spray pattern is produced with tails. The pressure should increase until the fan forms without any tails and thereafter increased by another 10%.

This represents ideal conditions and allows for line pressure drops, changes from climbing scaffolds and some latitude for viscosity changes mix to mix. The critical statement - 'atomise' - the lowest pressure necessary to atomise the material.

Paint needs to be properly mixed, inspected for obvious problems such as skinning. It remains good practise, because usually the volume required is high, to premix and box the paint from drum to drum ensuring colour and thickness consistency. Usually materials require between 5 and 10% by volume thinness during mixing. More than this can seriously affect the end results.

The Product Data Sheet will describe the paint material and spreading rate (SR), dry film thickness (DFT) and volume solids (VS). By applying these known factors, not only can the costs be predicted, the spray operation can be planned and executed with economy and accuracy.

The operator, having set up all the systems, including the paint and adjusting the spray properly, can apply the paint. There are two checking methods to measure the result as you work.

By matching area painted with volume used. This is simple providing the operator knows and can set the work properly. For example a material covering 8 square metres per litre will spread around 30 square metres for 4 litre and 80 square metres for 10 litres. Caution - one must allow for the volume of paint from the paint pump to the gun, which can be quite high. The advantage of this simple check system is that the spray operator can rapidly visualise areas against material volumes and form good working estimates.

The second system is to measure the wet paint thickness as work proceeds. The wet paint consists of the volume solids plus solvent spread at the correct rate. Because we need the wet paint to dry to an acceptable planned thickness, we must calculate the application thickness.

WFT (wet film thickness)
= 1000 divided by Spreading Rate (in square metres per litre)

In other words, a spreading rate of 8 square metres per litre needs a wet film thickness of 125 µ.

The measurement is taken by the painter with a calibrated hand held gauge pushed into the wet paint and the result read off. This again is simple to do and the operator soon learns the systems of setups that reliably return great results at very economical inputs.

So there it is, airless spray application can, and should, be carried out exactly in accordance with the manufacturer's recommendations. If this is done it is in harmony with all the other job inputs.

This information is drawn from the course notes provided to those who have attended Professional Painter Development Courses.

› View the Resene Getting the best out of your spray equipment information (for airless spray).


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