AW Extra 5/16/13 – Basic HVLP Spray Techniques

I made a lot of mistakes along the way, however. Spraying is a whole craft in itself, and I had to learn on my own, the hard way. If you’ve never sprayed before, or are just learning how, this step-by-step guide will help you avoid the most common pitfalls. We’ll cover how to set up your shop, how to adjust the spray gun and how to go about spraying a basic cabinet. Obviously there are a lot of fine points that we won’t cover, but this is really all you need to get going.

A good HVLP system can cost $600 or more, but if you want to take a test drive before buying, or simply want to get one big project done quickly, you can rent an HVLP system at a paint store.

Waterborne finishes are user-friendly

Waterborne finishes are clearly the best choice for spraying in a home shop.You get most
of the benefits of sprayed lacquer or polyurethane without needing an explosionproof
exhaust fan or explosion-proof lights.Waterborne finishes generally dry to the
touch in 3 to 10 minutes, and you can re-coat in less than an hour, building up 3 or 4
coats in a day. The quicker a finish dries, the less likely dust will contaminate the surface.
Clean-up simply requires warm water.

Be sure to buy a waterborne finish that is labeled for spraying. Waterborne finishes for brushing are too thick to successfully spray, and generally can’t be thinned.

Prepare Your Shop

Spraying in a small shop is a constant battle
with dust and overspray. (Overspray is the
fine mist that bounces off or misses your
project.) Outfit your shop with plastic
sheeting on the walls, a canvas tarp on the
floor, a halogen light and a window fan (about
$150 total). Dust kicked up by the gun can
ruin a finish.The tarp and sheeting eliminate
the dust hazard and keep overspray from
settling on your tools, floor and walls.Vacuum
your project before you spray.

Ventilate your shop with a window fan
behind the project, blowing to the outside.
Crack open a door or window on the other
side of the shop to supply fresh air.Wear a
respirator with an organic vapor cartridge
when you spray waterborne finishes.

Strain the Finish

Waterborne finishes may contain solid clumps that will clog the gun or spoil a finish. Solid pieces of finish or dust may fall into the finish can, too. When you pour finish into the sprayer’s cup, strain it through a small nylon anklet sock. You can rinse the nylon with water and use it over and over.

For the steps ahead you’ll need a large disposable surface to practice on. We used builder’s rosin paper, available at a home center for about $14 a roll, but plain brown paper or cardboard would work.

Practice Holding the Gun

There’s a right way and a wrong way to hold the gun, and you must be familiar with the right way before adjusting the gun’s controls.

The first step is to turn on the HVLP turbine to pressurize the cup. Pressure keeps finish from traveling back up the pressure tube and dribbling out of the gun.

Pick up the gun and hold it at 90 degrees to the surface. Don’t let the gun droop, particularly when you adjust the gun’s controls in the steps ahead, because that will create an uneven spray pattern. Hold the gun 6 to 8 in. away from the surface.

Lock your wrist and move the gun back and forth with your arm. The speed of your arm movement determines how much finish you apply. Move too slow and the finish will pool. Move too fast and the coat will be too thin. You’ll figure out the right speed after you set the gun’s controls.

Know the Gun’s Controls

Every HVLP system has three basic controls: the volume of air flow, the volume of fluid flow and the type and size of the fan pattern. These controls may be located in different places than on the gun shown at right. If you’re in doubt, check the owner’s manual.

An HVLP gun uses air to break up a fluid finish into tiny droplets. It’s called “atomization.” Once atomized, the droplets are formed into an adjustable spray pattern, called the “fan.” The trick to adjusting the gun is to get the right atomization, or balance of air and fluid volume, across the entire width of the fan pattern. But before you tackle that problem, the first control to adjust is the fan pattern itself.

Set the Air Cap for Fan Size

A small detour here. If you have the type of gun that has an adjustable air cap, set it now. Turn the cap so the nozzle is flush with the face of the air cap. As you become better at spraying, you can readjust the air cap to change the size of the fan pattern. For now, leave the nozzle flush with the air cap.

Select the Fan Pattern

All air caps can be set in three positions. Each position creates a different fan pattern. For learning how to set the air and fluid volume controls in the steps ahead, set the air cap to make a vertical pattern. Air and fluid volume settings are about the same for vertical and horizontal fans. Round fans require readjusting these settings.

A vertical fan is good for tops, or any time you move the gun side to side. A horizontal fan is good for sides, or any time you move the gun in and out, directly away from you. A round fan is used in special situations to concentrate the finish in a small area.

Too Little:

To start out, turn down the air control most of the way. Pull the trigger and make a pass. If finish spits out of the gun or if you see splatter at the edge of the fan pattern, turn up the air a little bit.

Just Right:

Slowly increase the air volume until the fan is evenly atomized and has clearly defined edges, without splatter.

Too Much:

If you turn up the air volume too much, you’ll get spray that looks like a fine mist, plus an excessive amount of overspray. Reduce the air volume and test again. (If the air volume is at the maximum and the gun is still spitting finish, use a thinner finish.)

Too Little:

Begin by adjusting the trigger for a minimal pull. As you move the gun, the fan should be smaller than 6-in. wide. Some spraying situations, such as spraying a narrow face frame, call for a small fan in order to minimize overspray.

Just Right:

Increase the fluid volume until the fan is 6-in. wide. Turn the knob to stop the trigger at this point. Now you can simply pull the trigger all the way back, like an on/off switch, to spray the same size fan. As you move the gun, you should get an even, wet coat of finish, not a mist and not a flooded surface.

Too Much:

Back off the fluid volume if your surface is flooded with finish, or move the gun at a faster rate. The fan will be too large when you pull the trigger back too far.

On an actual cabinet, remove the back if you can and finish it separately.

Spraying Face Frames and Small Surfaces

1. Switch the air cap to a horizontal fan pattern. Reduce the size of the fan pattern to about 3-in. across to minimize overspray. You can reduce the fan by reducing fluid volume, reducing the air and fluid settings together, or moving the gun slightly closer to the surface. Always test on paper before spraying your cabinet.

When you first pull the trigger to spray any vertical outside surface, such as this face frame, don’t aim right at it. Instead, aim the gun at a point above the face frame, as indicated by the yellow oval at left. Then pull the trigger and work your way down. Don’t release the trigger until you’re past the bottom. Do this in one fluid movement, always holding the gun perpendicular to the face frame.

Runs, Drips and Errors:

Yuck. Trying to be careful, I moved the gun too slow while spraying the side of a cabinet. Or maybe my fluid volume was too high, I don’t know. In any case, I let the mess dry, shaved off the excess with a utility knife and then sanded. Next time I’ll check my adjustments by spraying on a piece of paper first and make sure I keep the gun moving.

Missed Overlaps:

Cheapskate! What was I thinking? That I’d save finish by hardly overlapping at all? It’s not worth it, because here I’ve got a rough looking texture between the spray coats. Actually, I just couldn’t see where the fan pattern hit the surface, so I couldn’t judge the overlaps properly. What I really need next time is a bright, low-angle light.

Bubbles in the Finish:

I told everyone I really wanted the finish to look like Mars! Unfortunately, no one bought it. Bubbles mean the coat is too thick. Air got trapped below the finish as the surface dried. I waited until the finish hardened, then sanded out the offenders. To spray a lighter coat, I’ll either increase the air flow, decrease the fluid volume or move the gun faster.

Pools of Finish at the End:

There’s a wading pool-size puddle at the end of this top. What happened? I must have started this pass by aiming right at the end, and then pulled the trigger. I know that’s not the right technique, so I’ll repeat to myself as I move my arm, “aim at a spot just beside the top, and then pull the trigger.” In fact, I vow to keep practicing until it becomes second nature!

Clogged Needle:

Now what? My gun quit on me after I took a coffee break! No, the gun’s not at fault. While I was relaxing, finish dried on the cap and formed a hard bubble over the needle. No problem, I can clean it right off with a toothpick. Now I always check the cap for dried finish if I put the gun down for a few minutes. It’s as much a habit as taking coffee breaks!

Sources

Note: Product availability and prices are subject to change.

Sprayable Waterborne Finishes

These waterborne finishes are specifically
formulated for spraying with an HVLP system. This is just a sampling;
there are many others on the market.

Apollocoat AquaClear, advmachinery.com, 800-727-6553.

Crystalac, crystalac.com, 631-207-0328.

EF
Pro-series, generalfinishes.com, 800-783-6050.

Enduro
Wat-R-Base, compliantspraysystems.com, 800-696-0615.

Hydrocote, hydrocote.com, 800-229-4937.

PianoLac, pianolac.com, 845-855-0996.

This story originally appeared in American Woodworker July 2003, issue #101.

July 2003, issue #101

Purchase this back issue.