Popular Woodworking 2001-12 № 125, страница 55

tem is the better choice because it's less expensive, more mobile and safer to use.

What Affects the Finish Quality?

No matter what spray system you're using, many of the same concerns exist to produce a good-quality finish: humidity, air temperature during spraying and keeping the system clean.

Spraying during periods of high humidity (over 40 percent) and high temperatures (over 70 degrees) can result in a poor-quality finish by causing the material to dry either too fast or too slow. By adjusting the material-to-carrier (lacquer-to-thinner) ratio you can adapt to atmospheric conditions.

Many HVLP systems provide a viscosity cup to determine how much to thin the finishing material for optimal spraying conditions. First-time users should try the viscosity cup to get an idea of what "thin" is. Play with the mixture a little to get a feel of your gun's capabilities. Honestly, most furniture finishes (not paint, mind you) spray adequately through the standard nozzle provided with most guns without thinning. Eventually you'll put the cup away. I haven't used mine in years.

No matter how experienced you are at spraying a finish, if the fluid passageways in the cup and the gun (especially around the fluid nozzle) aren't clean, you're headed for trouble. The jet nozzles control the amount of air atomizing the fluid, as well as adjusting the shape of the fan (the spray shape). If those nozzles are clogged or partially blocked, the finish will not apply correctly. Another critical area to keep clean is the little hose leading from the cup to the gun itself. If it becomes clogged, air will blow through a bleeder gun but no material will be added. The best advice is to clean the gun and cup after each use.

You can probably leave the material in the cup overnight, but make sure you back the material out of the gun by lifting the gun out of the cup and pulling the trigger (with the turbine off) to allow the fluid to drain out of the gun. This isn't perfect, but will help overnight.

Even with the reduced amount of over-spray from an HVLP system, it's best to work in a place that is either well venti-

There are several critical adjustments on an HVLP gun.The fluid control knob regulates how much liquid is allowed into the airstream at the nozzle.The ring around the nozzle controls the width of the fan.The fan pattern in most guns is controlled by rotating the horns.The airflow is regulated either at the hose or on the gun.

lated, or equipped with a spray booth of some type (see Flexner On Finishing for a simple home exhaust unit). Unlike high-pressure systems, you can use an HVLP system in your driveway without concerns of overspray (though a respirator is still a good idea). Of course, if you're spraying outside, there are trade-offs. A slight breeze will pull the slight overspray away from you and the work, but it may blow bugs and falling leaves onto your project.

What Materials Can I Spray?

You can spray all types of top-coat finishes with an HVLP system, including paint, shellac, lacquer and polyurethane. However, most home woodworkers don't have a shop that is adequately ventilated or exhausted for some of these materials. Unless you are using a tested and approved spray booth with explosion-proof accommodations, I recommend only spraying water-based finishes in an HVLP system.

HVLP systems are capable of spraying stains as well, but spraying stains is a little more complicated than spraying a clear finish, and the clean-up usually isn't worth the mess (unless you're doing production work, like an entire kitchen). I'd recommend ragging or brushing on the stain and saving the HVLP for the top coat.

As with most finishing products, I recommend you choose one type of finish and learn to work well with it. Lacquer is my preference and, again, unless you're properly outfitted, use a water-based lacquer.

DE-'M1ST1FY1NG' SPRAYING

I started to try and explain how a liquid finish is turned into a mist by a spray gun, but then I read Bob Flexner's description from his Understanding Wood Finishing book and I thought I'd let him do the talking:

"Spray guns shoot a stream of fluid that is broken up into a mist of tiny droplets by jets of air coming out of the air nozzle. The droplets hit the wood and flow together to make a smooth film.The breaking up of the finish into droplets is called atomization. It's important that the atomization be thorough, or the droplets won't flow together well.

"The trick to achieving proper atomization is getting the right amount of air striking the fluid as it comes out of the tip of the gun. If you have too little air, the atomization won't be great enough, and the finish won't flow together. It will cure looking like the surface of an orange; the effect is called orange peel. If you have too much air, the finish will dry before it hits the wood, producing a dusty look. This is called dry spray.

"The two air jets that direct the atomizing air have an additional function. Because they are placed 180° apart, in the horns of the air nozzle, they force the atomized air into an oval-shaped pattern called a fan.The fan is perpendicular to the line of the horns. By increasing the air flow through these jets,you widen the fan, so you can coat a wider area with each pass. By decreasing the airflow, you shrink the fan to a very small circular pattern, which you can use to fill in small defects. By rotating the air nozzle, you can change the angle of the fan relative to the gun."

The nozzle on either an HVLP or conventional spray gun works the same. It's just that the way the air is delivered to the jet nozzles is different. The photo above shows the nose of an HVLP gun.You can clearly see the jets described by Bob, as well as the fact that I've been using the gun and it's due for a cleaning...

78 Popular Woodworking December 2001