Whether expressly stated or merely implied,
the transfer efficiency of HVLP spray guns is often quoted at
65 percent. This suggests that 65 percent of the paint in the
cup goes on the part and only 35 percent escapes into the air
as volatile-organic-compound (VOC) pollution. The transfer efficiency
and resultant money savings, however, are more dependent on the
painter than the paint gun.
Recently, the Iowa Waste Reduction Center
(IWRC) at the University of Northern Iowa received grants from
the Environmental Protection Agency and the Small Business Administration
to identify practical, cost-effective ways body shops could reduce
emissions and material consumption through spray efficiency. They
discovered that achieving a high transfer efficiency of paint
is determined by more than the type of spray gun used.
The Study
The IWRC defined the “spray system”
as having five integrated components: the spray gun, the painter,
the paint, the spraybooth/room and the part being sprayed (target
area).
It’s a hollow claim that any particular spray
gun can achieve 65 percent transfer efficiency (TE). What IWRC
found, instead, is that “the spray system can achieve up
to 65 percent transfer efficiency with a particular spray gun
when all other component parameters are specified.” Of the
five component parts of the spray system, the painter’s gun-setup
skills and spray techniques made the most difference in transfer
efficiency.
The IWRC then developed a method to quantify
the transfer efficiency both before and after painter training.
The training program focused on spray-gun adjustment and advanced
spray techniques to reduce overspray while maintaining quality.
(The IWRC recognized that the final finish quality of the paint
work was critically important. Paint rework can cost the shop
three times the money to redo, and a vehicle that must be repainted
increases the shop’s operating costs, paint consumption and VOC
emissions. A top-quality automotive refinish not only has to reach
a specified mil build to remain durable, but it must also meet
the subjective “customer-acceptable” test for gloss
and smoothness.) During the research, the IWRC was able to produce
good-looking paint work while still reducing paint consumption.
Part 1: Spray-Gun Adjustment
Spray guns of any style are designed to be
adjusted by the painter to achieve optimum atomization under current
shop conditions. Shop conditions, of course, are constantly changing.
The painter must adjust the spray gun to accommodate the material
being sprayed, the properties of the target area and the temperature,
humidity and air movement for each job. The IWRC suggests that
you adjust your spray gun by adjusting the pattern size first,
followed by the fluid flow, followed by the atomization pressure.
Unfortunately, many autobody painters simply grab their spray
gun off the bench and start painting – no matter the shape of
the target, the flow rates of the paint or the booth temperature.
HVLP spray-gun patterns – as opposed to conventional-gun
patterns – generally have a larger wet area in the center and
less of the dry, fuzzy ring at the edges of the pattern. To adjust
the pattern, turn the horns of the air cap up and down so the
pattern sprays out from east to west, rather than north to south,
and shoot the pattern on a fresh sheet of white masking paper
each time before you paint. By examining that pattern in the horizontal
position rather than the vertical, you can tell much more about
the true shape and condition of the pattern.
Adjust the pattern size to match the target
shape. In other words, don’t try to paint 1-inch tubing with a
full-size pattern or a van roof with a 3-inch fan. Once the fan
is even on both ends and set to the appropriate size, adjust the
amount of fluid flow and strive to deliver enough fluid to suit
your painting style. In general, less is better. Large fluid openings
require large amounts of air to atomize, and more atomization
air means more overspray. You cannot, successfully, adjust HVLP
spray-gun pressures by sound, pitch or “kick back.”
You have to use an accurate air-pressure gauge and set the pressure
to the exact pound to get the best results.
To achieve high transfer efficiencies, all
these adjustments must be made every time you begin to paint.
The IWRC discovered that the amount of control the painter could
exercise over the gun adjustments had a big bearing on material
savings.
Part 2: Spray Techniques
During the course of this study, painters
who visited the IWRC facility brought their own spray gun or chose
something similar from among several types and brands available
on site. Each painter painted a 1987 Honda Civic hood and fender
– chosen because they represented typical collision-repair parts.
The parts were weighed to within one-tenth
of a gram before and after painting, and the paint loaded into
the spray gun was weighed to within one one-thousandth of a gram.
After the painter applied two or three coats of basecoat, the
parts and the spray gun were reweighed to establish transfer efficiency.
The IWRC checked the mil build in all areas of the panels to ensure
a quality job, and the painters were videotaped as they based
and cleared the hood and fender.
Seeing yourself on videotape vividly points
out flaws. I was one of the people the IWRC tested and trained,
and I was stunned to see the idiot in the video wave the spray
gun around. One basic lesson to improve transfer efficiency and,
hence, to save money, is to point the spray gun at the target.
Remember the part about not arcing the gun? I remembered it, but
the guy (me) on the tape held the gun much closer to the center
of the panels than he did to the edges. Keeping a steady 90-degree
angle to the panels and a uniform target distance make a huge
difference in the overspray cloud. To get the most paint on the
part, it’s necessary to look closely and steadily at the part
while triggering the gun. Good tennis players keep their eye on
the ball all the time; good painters need to watch the wet paint
edge on each pass on each panel.
Part 3: Paint Preplanning
In addition to the videotape, the training
session included the notion that the painter should have a plan
of attack before beginning to spray. About the only time most
painters walk through or think through a paint job is when spraying
candy colors. Translucent candy or pearl colors will show dark
streaks each time they’re overlapped, necessitating a careful
preplan and a continuous walk down the entire side of the car.
The IWRC, however, got me to rethink the conventional
wisdom of car painting. For example, I painted the narrow back
portion of the Honda fender by passing the gun back and forth
east to west from the bottom edge of the fender to the top. After
training, I simply turned the pattern sideways and painted the
same part of the panel in one pass, south to north. By thinking
about how to orient the pattern to put the most paint on the target
part, substantial material savings can result.
One technique change I’ve adopted successfully is to overlap the
edges of the panels much less. For example, the Honda parts were
laid out on white masking paper. Once the part was removed, overspray
outlined the shape of the part. Before training, my overspray
band was 4 or 5 inches wide in some spots. After training, that
same outline was only 1 or 2 inches wide. What an obvious change
a little less spray-gun lead and lag distance made. Simply pointing
the gun so that 80 or 90 percent of the spray pattern hits the
part and only 10 percent lands on the masking or flies off into
the air will make a big difference in paint usage.
Back at Your Shop …
By measuring the mil build on your own paint work for a week or
two, you may discover that you’re applying more paint than necessary.
If you use the same brand and type of finish routinely and if
you spray it through the same equipment, the mil readings from
this sample period will be representative of your paint work.
If the paint manufacturer calls for a 2- to 3-mil film build and
your work is 5 to 6 mils thick, then you’re spending money unnecessarily.
The other side of the coin is you may discover you’re not applying
sufficient mil build, which will affect durability.
The IWRC suggests you write down and record how much paint you
used for each part. One often-repeated problem with an old painter
and a new HVLP gun is that there isn’t a big material savings
at the end of the first month with the new gun.
Many times, though, the problem is the amount of paint mixed for
the job. Here’s a painter with years of experience who knows exactly
how much color to mix up, except that the new gun sprayed the
same part with 40 percent less paint. The leftover paint ends
up in the hazardous-waste drum for a net savings of zero to the
shop. Simply writing down exactly how much paint was required
to spray a week’s worth of assorted parts with color and clear
will go a long way toward reducing the shop’s paint bill. If your
shop throws out as little as one pint of color a day, at the end
of the year, you’ve tossed out around $3,000.
In the Hands of the Painter
The design of the particular spray gun, the physical conditions
of the shop, the quality of the compressed air and the characteristics
of the car part all have a bearing on transfer efficiency. The
biggest pollution reduction and corresponding material savings,
however, are in the hands of the painter.
Going into the IWRC’s study, I was pretty sure I could outsmart
the process. After all, I knew about HVLP guns, understood about
adjusting the pattern and generally was positive I could dazzle
their research with my high transfer efficiency.
Wrong.
After painting the two Honda parts with no instruction, then watching
my videotape and hearing their plan to adjust all the variables
available to me, I painted another identical hood and fender with
the same paint and the same spray gun. How’d I do? When the new
parts were weighed after spraying, I had painted the same parts
with almost 50 percent less color and clear – proving, once again,
that the best spray gun in the world is only as good as the painter
operating it.
Adjusting to HVLP
To get the most from your HVLP gun:
- Add a slower-evaporating solvent to your paint mix. Since
HVLP guns produce a larger paint-droplet size than conventional
suction-feed guns, the solvent must stay in the paint film longer.
With a slow-evaporating (warm weather) reducer, the larger droplets
dry slowly enough to melt into one another and flatten out. Using
a fast-dry reducer with HVLP means a greater risk of an orange-peeled
finish because the droplets dry too quickly and don’t level off
flat. - It’s poor practice to spray the first coat as a tack coat
for the same reason (larger droplet size). Years ago, when the
industry was shooting synthetic enamel paints, the painters learned
to spray a light coat on the first pass, which acted like “glue”
and helped to keep the subsequent coats from sliding down the
vertical panels; if you shoot a tack coat with HVLP guns, however,
it’s very difficult to reflow the dry-sprayed first coat with
the next coats. - You need a slower hand speed and a closer gun distance – much
different than painting with conventional siphon-feed guns.
Reaping the Rewards
When used correctly, HVLP technology can improve your shop’s productivity.
The benefits include:
- governmental compliance. If your locality mandates HVLP spray
guns in your shop, you are, no doubt, subject to a stiff fine
if you fail to comply. - cleaner paint work and a healthier spray environment. Without
high atomization pressures, the dust in the drip rails tends to
stay hidden. Also, high air pressure blows all the dust and dirt
from every nook and cranny, which then lands back in the wet paint.
With HVLP, low overspray clouds keep the spraybooth a lot clearer;
if the painter puts the paint on the vehicle rather than blasting
it up into the air, it’s much easier for him to see what he’s
doing inside the booth. Not only can the painter see the car better,
but the air in the booth isn’t laden with an evil haze of unhealthy
isocyanate vapor. - a demonstrable labor savings in masking time. Since the overspray
doesn’t go as far, less masking is required to keep the paint
out of unwanted areas. - the potential for paint-material savings. This HVLP spray-gun
benefit appeals most to shop owners because every shop owner wants
to save money.
