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.
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.
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
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
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).