The Basic Concepts Of
Adjusting An HVLP
And Some Tips On Cleaning A Spray Gun
Bruce W. Maki,
Spray finishing with an HVLP spray
gun can be highly productive and give superior results, or
it can be frustrating and quickly ruin a painting or wood finishing
The key, I believe, is understanding all the
variables involved (some of which are adjustments to the
spray gun) and having enough patience to make small
adjustments and observe the results.
Spray painting or spray finishing can be dangerous,
hazardous to your health, and cause serious property
This article should not be viewed as
"instructions" or "advice". This article
is for entertainment purposes only.
If you undertake any spray finishing project, then you
are working at your own risk. Under no circumstances
will HammerZone.com or it's publisher be held liable for
any damages caused by your actions.
Be aware that
I am not an expert on this
subject. If you chose to pursue spray finishing, I recommend
that you do additional research... I do not know
everything about this subject. See
Recommended Reading at the end of this article and read the
The Many Variables Of Spray Finishing:
Air Pressure Regulator:
The red arrow points to the air pressure
regulator adjustment knob. When the knob is
turned in (clockwise), the pressure
regulator will maintain a higher pressure, which
can be seen on the pressure gauge.
I usually set the
pressure regulator between 18 and 22 PSI. A higher pressure setting
results in better "atomization" of the liquid, which just means that
the spray mist is finer because the droplets are smaller.
However, higher pressure also results in more overspray.
Note that the air pressure
entering this spray gun cannot exceed 50 PSI, so I
adjust the pressure regulator on the air compressor
down to 50 PSI. Excessive pressure at the inlet side of the
gun can damage the gun's pressure regulator. Other spray
guns may have a different maximum pressure.
The Main Spray Gun
1: Air Volume Adjustment
2: Material Volume Adjustment
3: Fan Width Adjustment
1) Air Volume Adjustment:
When this knob is
turned fully clockwise, the air flow is shut off. It takes some
experimenting to find the right air flow setting. Too much air flow
can make excessive overspray and cause a rippling effect in the
liquid that has already been applied to the surface. I understand
that excessive air flow can also cause the wet surface to dry too
fast, which can create problems in the final results like blush.
Too little air flow can result in improper
atomization, which may be noticed as spots of liquid on the surface.
You may notice that the gun is "spitting" liquid instead of making a
nice cloud of spray.
2) Material Volume Adjustment:
This knob is on
the top rear of the spray gun. Turning the knob clockwise closes the
valve and reduces the amount of liquid that flows from the cup into
the gun. If the material volume is too low, the spray stream will be
weak and it may take a long time to apply the finish. If the
material volume is too high, the problems of inadequate air volume
may show up because you are basically trying to atomize too much
liquid for the volume of air flowing through the gun.
Note that the air volume and material volume
can be adjusted together. By that I mean there may many
settings of air volume and material volume that will give excellent
results. When both valves are opened slightly, you basically have a
low capacity spray gun, which could give results similar to an
"airbrush". An airbrush is essentially a small-volume sprayer for
fine detail work. In theory you can use an HVLP sprayer in place of
an airbrush, but the tool is much heavier so it's much more
difficult to control and get the precision results that are possible
with an airbrush.
When both air and material volume are
increased, the spray gun can reach it's maximum capacity in how much
surface can be covered per unit of time. Open up these valves a lot,
and you can spray a lot more square feet per minute. But either air
volume or material volume may still need to be tweaked to give the
3) Fan Width Adjustment:
This control, which is
usually on the side of the gun body, adjusts how much the spray
stream is "flattened" into an oval shape. With the fan control fully
off (turned clockwise) the spray stream will create a round spot. A
wide oval spray pattern lets you cover more area quicker, but if the
fan is too wide the droplets of liquid may not "melt" together
properly, resulting in a lumpy surface commonly called "orange
If the fan width is set too small, it will
take more side-to-side passes to cover the area being sprayed, but
the bigger problem is the fact that it's easy too apply too much
liquid and have problems with runs and sags.
Note that the fan spray pattern can usually
be adjusted from vertical to horizontal, or any angle in between,
simply by loosening the air cap (the removable cover on the front of
the spray gun) and rotating the nozzle. I usually leave the spray
pattern at vertical, but a horizontal pattern is useful if I'm
spraying something tall and narrow.
Tip: When I'm adjusting my spray gun,
I try to get a cloud of mist similar to what comes out of a quality
can of spray paint or urethane.
There are other "adjustments" involved in
spray finishing that don't involve the spray gun.
Thinning Of The Liquid:
When the liquid is thinned (diluted) with a
solvent it's easier to atomize, but thinning also changes the speed
of drying which has a whole set of consequences. Too much thinner
can also create problems in the final results, like blush or
"solvent pop". I understand that coatings like
polyurethane should be thinned (with mineral spirits) no more than
10 per cent.
Distance From Spray Gun To Work Surface:
advice I've heard is to keep the spray gun tip about 12 inches (30
centimeters) from the surface being sprayed. However, this distance
can be altered, and sometimes it's difficult to maintain the desired
distance. If the distance is too great, the droplets of liquid can
start to dry before they hit the surface, and then droplets won't
melt together, leaving a pebbly surface or orange peel.
If the distance is too close, the coating
may be too thick, which can cause runs and sags and other problems
from drying too slowly. I've read that the air flow from the gun may
create problems (like rippling) when the distance is too close, but I haven't
experienced these problems.
Speed Of Travel:
How fast you move the spray
gun from side to side will effect the amount of liquid applied on a
given area. Slow travel means a heavier coat. Fast travel means a
lighter coat but possibly problems with droplets blending together.
When spraying stain, the results may be a speckled appearance. In
theory, too fast of a travel rate could create a lumpy appearance in
urethane or paint, because the droplets are too far apart too blend
together. I can't say that I've ever had a noticeable problem from
moving too fast when I've sprayed urethane or automotive paint. In
general, too fast of a travel rate seems to cause fewer problems
than moving too slow.
Amount Of Overlap Between Passes:
The general rule is to make each pass
overlap the previous by 50 per cent. Easier said than done. I don't
use my spray guns very often, so I get out of practice. Sometimes it
can be difficult to keep the overlap constant, or to achieve that 50
per cent overlap.
Too much overlap will apply a heavier
coat, which might cause runs and sags and other problems.
Too little overlap can create bands
or streaks where the material isn't thick enough. This is a problem
when spraying paint or stain, it's not a problem when spraying clear
finishes like urethane because you can always cover that flaw with
the next coat (unless it's the final coat).
Ambient Air Temperature:
Ideally, spray finishing should be done
around room temperature. When this is not possible, some
professional painters will substitute different solvents when
diluting their coating.
In cold weather, xylene (also called
Xylol) can be used as a thinner. Xylene (pronounced "zye-leen")
is quite volatile and evaporates easily. Xylene is a common
ingredient in carburetor cleaner and is highly flammable.
In hot weather, VM&P naphtha can be
used as a thinner. Naphtha evaporates slower than mineral spirits,
so in hot weather it may help prevent the coating from drying too
fast Naphtha is used for lighter fluid in old-fashioned lighters
(such as Zippo lighters). If you've ever used such a lighter, you'll
may have noticed that naphtha doesn't burn as fast as other solvents
or fuels. Both xylene and naphtha can be purchased at paint stores,
hardware stores or major home centers.
Personally, I have not tried using these
alternative solvents. I would prefer to wait for better weather to
do my spray finishing. This is considered an advanced technique and
should be approached with caution.
Tips On Cleaning An HVLP Spray Gun:
When I'm done spraying, I will pour the
unused liquid back into it's container. Then I pour some solvent
into the spray gun cup, swish it around, and connect the gun to the
air hose. I then spray against a piece of paper or cardboard for a
few seconds until I can see the solvent spraying out.
Then I disconnect the air hose and take the
spray gun outdoors for cleaning, if possible. I typically put the
spray gun in a plastic dishpan, empty any solvent from the cup, and
then dismantle the gun for cleaning.
part I remove is the air cap.
can be removed by hand.
Wearing chemical-resistant gloves, I will
soak this in solvent and clean it with a small
stiff-bristle brush (with plastic bristles, not
Then I remove the nozzle with the multi-wrench
(included with the spray gun). On both of my
spray guns this requires the 19mm wrench
loosened, the nozzle can be unscrewed. It helps
to pull the trigger while doing this, which
retracts that needle.
I soak this part in
solvent and clean it with a brush.
Then I unscrew the material cup, which usually
requires that multi-wrench.
I usually clean
the cup last.
Under the cup
is a little screen filter.
I remove the filter
and clean it in solvent.
Replacement filters should be available at
most auto body supply stores.
After I remove the filter, I hold
the spray gun over the dishpan and pour some clean solvent
down that filter hole.
Then I lay the gun in the solvent and use a
small bottle-brush to clean the areas indicated
by the arrows:
- The liquid passageway
- The threads for the air cap
- The inside area around the needle.
Be careful... if you bend that needle it will
need to be replaced, along with the nozzle.
After rinsing everything with clean
solvent, I blow all the parts dry with compressed air, or at
least wipe everything dry with paper towels.
I pour the used solvent into a container
marked "used paint thinner" or whatever solvent it happens to be.
Used solvent should be treated as hazardous waste and disposed of
Warnings About Do-It-Yourself Spray Finishing:
Whether applying paint, stain,
urethane or lacquer, there are some bad things that can
happen when spray finishing.
Explosion and/or Fire:
Many wood finishes are oil-based and use flammable
solvents such as mineral spirits, acetone or lacquer
thinner. When these finishes are sprayed, the solvent
vapors can be explosive. Keep away from open flames or
hot surfaces such as light fixtures. Spray
finishing requires lots of ventilation.
Health Risks Of Paint Fumes:
Petroleum solvents can make you feel light-headed, but
the bigger problem is that solvents dissolve the fatty
myelin around your nerve cells. Myelin acts as
electrical insulation for your nerves and brain, and
when this stuff dissolves your brain can
short-circuit. You wouldn't melt the insulation
on the wiring in your car, would you? Don't do it to
If you spray finish indoors,
wear an organic vapor respirator. I bought one for
about 40 bucks at Home Depot. These things use activated
charcoal filters to absorb all the volatile organic
compounds that pass through them. Eventually they get
plugged up and the filters need to be replaced. I store
my respirator in a sealed Ziploc bag when not in use.
Overspray: Even with an
HVLP (High Volume Low Pressure) spray gun, there is
still plenty of overspray. If spraying indoors,
the overspray can cover large portions of your work
area. If spraying outdoors, the overspray can
fall on cars parked nearby. If you cover your neighbor's
house or car with paint overspray, you may be held
liable for damages. Sometimes overspray wipes off like
dust, but sometimes it sticks really well. Watch the
direction of the breeze, and observe how far the
overspray is travelling.