62 ■ POPULAR WOODWORKING
Air Cap
Air caps are also commonly sold
together with fl uid nozzles and
needles as a set. But you can inter-
change air caps and fl uid nozzles to
improve atomization. If your source
of air is a compressor, you can also
use a diff erent air cap to widen the
spray pattern or fan width.
The air cap contains a number of
holes to atomize the fl uid and shape
the pattern. The air that comes
through the center hole and other
holes next to it atomizes the fl uid.
The air that comes through the hole,
or holes, in the horns shapes the
pattern. The larger these holes the
wider the possible spray pattern as
long as there is enough air.
Making Choices
So how would you mix and match air
caps and fl uid nozzles to improve the
quality or effi ciency of your work?
There are fi ve variables: viscosity of
the fl uid material; the amount of air
your compressor or turbine produc-
es; the speed at which you want to
spray; the size and shape of the ob-
ject you’re spraying; and the quality
of results you want.
The thicker the liquid you’re
spraying, the larger the fl uid-noz-
zle orifi ce you should use because
it’s harder to push thicker liquids
through small holes. Smaller orifi ces
will cause you to have to move the
spray gun more slowly over the
surface. The exception is the pres-
sure-pot setup where you could turn
up the pressure to compensate.
It’s obvious that you have to have
enough air from your compressor or
turbine to atomize the liquid you’re
spraying to reduce orange peel and
move at the speed you want or you
won’t get good results. You can com-
pensate for too little air, however, by
using a smaller fl uid nozzle and air
cap or by thinning the material so it
doesn’t take as much air to atomize
it. Either way will result in your build
being less.
If you want to increase your fl ow
rate, or the speed at which you can
move over the surface, you will need
to use a larger fl uid nozzle and air
cap, as long as you have enough air,
of course, to get good atomization.
Your effi ciency or ability to reduce
waste is determined by the size and
shape of your work. For example,
if you are spraying a large tabletop,
your effi ciency is increased by using
a needle/nozzle set and air cap that
gives you the widest possible fan
width for fewer passes to cover the
surface. By contrast, if you’re spray-
ing chairs with narrow legs and arms,
you want a narrow pattern to reduce
overspray. You can achieve this in
two ways. One is to use a needle/
nozzle set and air cap that gives you
that narrow pattern. The easiest way
for most amateurs and small shops
is simply to close down the fl uid and
fan-width control knobs, usually
located at the gun’s back.
The quality of your work is mea-sured by the amount of orange peel
you get. The less obvious the orange
peel the higher the quality. Orange
peel is usually caused by spraying
too thick a liquid with too little
air or by moving the spray gun too
rapidly across your work. To correct
the fi rst problem, increase the air
pressure (from a compressor) and
change to a larger needle/nozzle
set and air cap, or thin the materi-
al. To correct the second problem,
slow down or thin the material.
You won’t be able to increase the
amount of air with most turbines.
For most projects you can proba-
bly get by with the needle/nozzle set
and air cap that came with your gun.
But it’s good to know that you have
more control if you want. PWBob Flexner is the author of Flexner
on Finishing, Wood Finishing 101, and
Understanding Wood Finishing.3
45
Spray Guns
3 Needles and fl uid nozzles are usually sold in sets. Some manufacturers mill
their needles to a more gradual taper so they can be used with several different
sizes of fl uid nozzles. These fl uid nozzles are 1.3mm, 1.4mm and 1.5mm.
4 The size of the hole in fl uid nozzles makes it easier or harder to get thicker
liquids, such as paint and adhesives, through. Smaller hole sizes require more
air pressure or thinning of the fl uid material to get it through and atomized well.
5 The hole in the center of the air cap and the hole, or holes, to the side atomize
the fl uid. The holes in the horns are smaller on high-pressure air caps (left) and
larger on HVLP air caps to let more air under very little pressure through.