Popular Woodworking – October 2019

60 ■ POPULAR WOODWORKING

Flexner on Finishing

PH

OT

OS

BY

TH

E^ A

UT

HO

R

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.