60 ■ POPULAR WOODWORKING
Flexner on Finishing
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As I have said many times,
spray guns are pretty simple tools.
By comparison, they aren’t any more
diffi cult to use than a router. Still,
there are a couple of critical parts
you should be familiar with to get
the best results. These are the fl uid
nozzle and the air cap.
NOTE: This article doesn’t cover airless
or air-assisted airless spray guns. Airless
are popular with painters who want to
cover large surfaces quickly and don’t
mind the small trade off in quality, and
air-assisted airless are popular in indus-
try and large cabinet shops that have a
high volume of work to be fi nished.
Introductory Remarks
In the last issue of Popular Wood-
working (#247), I explained the ba-
sics of spray guns and how to choose
which to buy. Here’s a short review
of the article.
Spray guns work by using air to
turn a liquid stream of fl uid (paint,
fi nish, stain, glue, whatever) into a
mist. This is called atomizing. There
are two sources of air: compressors
and turbines. Compressors produce
air under high pressure. Turbines
produce high volumes of air under
low pressure. Both can be used to
power HVLP (high-volume-low-
pressure) spray guns. These are the
newer type spray guns that came
on the market about 30 years ago
and are now so widely used that it’s
become diffi cult to fi nd the older
high-pressure spray guns.
The advantage of HVLP is equiva-
lent quality atomization at much low-
er pressure, so there is less bounce-
back and therefore less waste.
Fluid Needle and Nozzle
The fl uid needle and nozzle are sold
as a pair, though with some brands
one size needle can be used with
several size nozzles. This is accom-
plished by milling the needles to a
more gradual taper. Either way it’s
critical that the needle “seat” well
into the orifi ce of the nozzle so it
doesn’t leak.
The purpose of the fl uid nozzle is
to control the amount of fl uid that
exits the spray gun. Larger orifi ce
nozzles allow more fl uid to exit and
also allow for thicker materials,
such a paints and adhesives, to be
sprayed. Smaller orifi ce nozzles
restrict the fl ow. Typical sizes used
for most stains and fi nishes range
from 1.8 mm (0.07 inches) to 0.5
mm (0.02 inches).
Off -the-shelf spray guns usually
come with a 1.4 mm (0.05 inches)
nozzle. You may fi nd that this
works well with everything you
spray. It usually does. But if you
want more control, especially if
you spray a variety of surfaces or a
variety of liquid materials, you may
want to use diff erent size nozzles.
Better quality spray-gun manu-
facturers off er fl uid needles and
nozzles in a number of sizes.
1
2
Getting the Most From Your Spray Gun
By Bob Flexner
The critical parts are the fl uid nozzle and air cap.
1 On most spray guns
the fl uid and fan-width
control knobs are on the
backside of the gun. The
fl uid control knob is the
lower one in this example.
It always lines up with the
fl uid nozzle and needle.
2 Sometimes the fan-
width control knob is on
the side of the gun rather
than the back.