How the box is built, where the moving air comes from and
where it goes on the way out of the box are the basic issues about
any booth. To achieve cleaner paint work, however, you must fully
understand these basic issues, along with the bells and whistles
– such as lights and doors – that make the booth easier to use.
The Building Blocks
Class I Division I locations where flammable
solvents are being sprayed generally require walls and ceilings
with a two-hour fire rating. The rating describes the ability
of the enclosure to contain flames, hopefully long enough to extinguish
them. Two-hour ratings are available by using metal panels, cement
blocks or even double-layer, 5/8-inch sheet rock on metal 2x4s.
- Cement blocks – One problem with cement-block construction
is the porosity of the blocks: OSHA regulations may call for "smooth"
interior walls. Block walls would then have to be filled and sealed
to obtain a smooth finish. Sealing the blocks also makes for cleaner
paint work, eliminating any place for dust to hide.
metal panels to construct the spraying box. Some booths use single-thickness
metal panels, and others use double layers with insulation sandwiched
For many years, rust was a huge problem for spraybooths, just
as it was for automobiles. Many booths were assembled on top of
a 4-inch cement curb or up on treated 4x4s. The goal was to keep
the bottom edge of the booth off the floor, where it would stand
in water and soon rust.
These days, galvanized metal is used for most spraybooth construction.
Just like the dramatic effect it’s had on eliminating rust on
automobiles, galvanized metal has also eliminated rust-through
on spraybooth panels.
The panels themselves are flanged to interlock flush on the inside
of the box, and most booths require caulking of seams and joints
when construction is finished. Tip: If you haven’t recaulked
your booth lately, this would be a good time. Don’t use the expensive
autobody seam sealer, though – instead, buy the inexpensive stuff
at the lumber yard. The finish on the panels runs the gamut
from unpainted galvanized to high-gloss baked enamel with stainless-steel
hinges and fasteners.
- Insulated metal panels – These are more expensive to
construct than single-thickness, 18-gauge, rolled-edge panels
because twice the metal is required (inside and outside), which
makes raw-material costs higher. On a positive note, double-sided
panels make for a quieter-running booth, and all that insulation
absorbs sound, too. Double-sided panels also travel better than
single-thickness sheet metal. You know how hard it is to get a
replacement quarter panel all the way from the manufacturer to
the technician’s stall without a footprint in it? It’s even harder
to ship sheet metal across an ocean without damage.
Proponents of double-wall construction also contend that more
heat will be retained inside the booth during any force-dry cycle.
The main heat loss, though, comes when the exhaust fan carries
the heated air away at 10,000 cfm, not through the metal sides
of the booth.
Where the air to be exhausted comes from is the next big variable.
For most body shop spraybooths, the exhaust fan will move out
about 10,000 cfm of air. For most shops, the replacement air is
simply sucked from inside the rest of the building.
When the room doors are hard to open and slam shut behind you,
it’s a tip that you don’t have enough replacement air available.
Also, when the fan is starved for air, the dirt problem inside
the booth gets much worse.
That fan wants 10,000 cfm every minute. If it can’t pull the required
air through the intake filters, it’ll pull it through the cracks
in the corners and around the door edges. And since the replacement
air is coming in from the rest of the body shop, it’s full of
dirt and dust.
The fastest way to cleaner paint work is to install an air-replacement
furnace on the booth. This device takes clean air from outside
the building, filters it, heats it (if necessary) and delivers
it right outside the spraybooth’s intake filters. Since there’s
much less dirt in this air, the intake filters do a better job
and last longer than when the replacement air was dragged across
every dirty car in the shop on the way into the booth.
It’s also advisable to bring in slightly more air than is being
exhausted to establish a positive pressure inside the spraybooth.
For example, if the exhaust fan moved 10,000 cfm of air each minute,
the air replacement should bring in 10,001 cfm of replacement
air; now the pressure is greater inside the booth than outside.
If you stood in front of the booth-door seam with a handful of
dust, the dust would be blown off your hand by the escaping air
– not sucked through the crack into the booth.
An air-replacement furnace to fit most automotive spraybooths
will cost about $10,000 to $15,000. An existing crossdraft booth
also would need an intake plenum, basically a metal box to hold
air. By installing a metal box in front of the regular intake
filters and ducting it directly to the air replacement unit, you
have a closed system. This makes for much cleaner paint work and
also offers the opportunity to speed production by force drying
Forcing the paint to lose solvent and crosslink quicker is possible
by raising the temperature in the booth to around 140 degrees.
Most air-replacement units are capable of either a 90-degree or
a 140-degree temperature rise; this means they can intake outside
ambient air at 20 degrees and, in one minute, raise 10,000 cfm
of air to either 110 degrees or 160 degrees, depending on how
large the burners in the furnace are.
Force drying the work in your current crossdraft booth will produce
more paint jobs by the end of the week, even without buying a
The direction of the airflow within the spraybooth can make a
huge difference in the quality of the paint work. In a standard
crossdraft-airflow design, the incoming air is dragged the length
of the vehicle before being exhausted. In a downdraft airflow,
the incoming air is passed over a single section of the car before
being immediately pulled out of the floor exhaust; any contaminants
in the air are gone before the paint can be
- Downdraft airflow – This airflow is the most desirable
choice for fast-drying, clean paint work. The vehicle dries faster
because the 10,000 cfm of rapidly moving (100-plus feet per minute)
air pulls the solvent out of the paint film very quickly. Note:
Add approximately 10 degrees to the booth temperature when choosing
a solvent for use in a downdraft because of all the moving air.
Getting the air to flow smoothly from top to bottom requires
an air-exhaust trench below floor level. This additional construction
cost can add several thousand dollars to installation expenses.
Some shops choose not to purchase a downdraft booth because they’re
in a rented building, but the cleaner paint work possible with
a downdraft will recover the cost of the pit construction quickly.
Not having to sand and buff a farm field out of every paint job
saves a ton of time, and when the time comes to leave the rented
building, a truckload of sand and a couple hundred dollars in
cement work will fill and cover the exhaust pit in about an hour.
If you absolutely can’t dig a pit, you can buy a plenum (box of
air) to sit beneath the booth to carry out the exhausted air.
In this case, you would also need ramps to drive the car up into
- Semidowndraft airflow – If you can’t make either downdraft
alternative work, consider a semidowndraft. This spray box is
installed at floor level, but the intake air comes in from the
rear third of the booth ceiling. The exhaust is pulled from the
front edge of the booth near floor level, which sets up a diagonal
airflow across the car. This drags less dirt by the vehicle than
a crossdraft does but passes over more of the car than a downdraft.
In any case where the air is delivered from above the vehicle,
special air-balancing intake filters are used. Make sure you use
replacement intake filters that meet the booth manufacturer’s
specs. Not all air-balancing filters are the same, and using a
more or less restrictive type will affect airflow dramatically.
Frequent changes of both the intake and exhaust filters are necessary
to keep the work clean.
Good, Clean Fun
Whatever the design of your booth, the dirt in your paint work
still comes almost equally from the car, the painter and the booth
itself. As many shops have discovered, just buying a heated, replacement-air
downdraft booth will not automatically provide clean paint work.
Successful painting requires the painter wear lint-free coveralls
and a fresh head sock. Prepping the car for cleaner work means
a soap and water wash with a drenching water rinse. Using treated
masking paper and even double masking the car will keep the work
cleaner, too. If you can’t afford a new booth, try a disposable
paper suit, a few extra minutes with the water hose and some high-quality
masking paper. Cleaner paint work doesn’t always require new equipment
– sometimes all it takes is some good old common sense.
Mark Clark, owner of Clark Supply in Waterloo, Iowa, is a contributing
editor to BodyShop Business.
Where the exhaust stack leaves the building, what size it is and
how high off the ground it climbs are often regulated by state
or local ordinances. New spraybooth permits are quickly becoming
the norm across the country, so begin any new booth project with
a call to the authorities first.
If you can’t exit the exhaust out the side of your building, you
need to know that before booth installation begins. If you must
use a particular gauge or diameter duct work, find out before
the contractor begins work. In fact, write down who (the booth
manufacturer, the jobber or the shop) will be responsible for
what. Every shop owner I ever talked with liked his new booth
when it was all said and done, but many had a trying time getting
it purchased, delivered and installed. Plan ahead to avoid the