Step Up to Spray-Finishing

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This article is from Issue 48 of Woodcraft Magazine.

Spray-Finishing

Step Up to Spray-Finishing

Weigh your choices for a system that meets your needs.

By Jim Harrold with Charles Neil

 

Woodworkers dream of the day when they can abandon their pad or brush and reach for a spray gun. Indeed, to achieve a showroom finish that feels as good as it looks, and particularly with larger furniture pieces, spraying may prove your best option for applying consistent color and a hardworking finish to lock it in. But before you take the big step, know what to look for so you can match a spray gun system with the type of woodworking you do, the amount of time you plan to spray-finish, and your budget. The good news is that if you own a capable compressor, you can get into spray-finishing for as little as $40. At the other end of the spectrum, for the guy who wants a top-of-the-line turbine-powered spray system, you can pay over $1,000. What are the differences, what do you get for the money when you spend more, and what systems lie between the extremes? Here’s a representative look at some HVLP spray gun systems and the choices that matter most.

What Does HVLP Mean?
HVLP stands for high-volume low-pressure and relates to a spray-finishing system that relies on a fan or vacuum blower-style motor or compressed air to move a dye, stain, or finish fluid from its pot or cup through a needle/ nozzle and onto the project. Combined with the proper spray gun, these systems can provide a finer atomization of the fluid using less pressure, which results in reduced overspray (less airborne particles). This, in turn, means more finish product adheres to the surface than with older high-pressure spray systems. Today’s HVLP “conversion” spray guns let you connect to a compressor to achieve similar results.

Spraying versus brushing

So why add a spray gun system to your finishing locker? Count the advantages:
• Spraying offers a level of speed and control impossible to match with brushing or padding.
• Spraying lets you add even color coats and move from a lighter to darker color for an overall desired look. Then it lets you lock in the color with a clear topcoat that doesn’t mechanically disturb the color underneath in the way brushing and padding can.
• It lets you coat nooks and crannies better and more easily, as well as the neighboring hard and soft grain in wood parts.
• It reduces the blotchiness associated with brushing or padding stain on some woods.
• It reduces the darkening that can occur on end grain when brushing or wiping on a stain.
• Spraying avoids brush marks, streaks, bubbles, or remnant brush hairs.
• Spraying’s speed lets you keep up with fast-drying water-based stains and finishes.
That said, trade-offs do exist, beginning with the initial equipment costs of spraying. Then there’s the need for a spray booth to contain overspray, and good ventilation, particularly when using solvent-based products. Compared to brushing, spraying requires that you wear a chemically approved respirator to prevent airborne finish solids from entering your lungs. Finally, cleaning a spray gun and pot takes longer than simple brush cleanup.

What Is Atomization?
Atomization is the process of breaking down color or finish fluids from larger to smaller droplets–the finer the spray, the smoother the coating. Adequate air pressure and guns with the appropriate needle/nozzles provide the best results.

Choices in spray system components 

Spray systems consist of four parts–an air power source, spray gun and components, pot, and hose. Let’s look at each individually.

Air Power:
Air Compressor–Up until the mid-1980s, compressors and conventional siphon-style spray guns were the only game in town. But with today’s HVLP conversion guns, you now have a more efficient system, resulting in less overspray and product waste. In order to spray the full range of finishing products, however, you need at least a 21⁄2 to 3 hp compressor with a 20-gallon tank. One this size lets you effectively atomize the product and supply constant air pressure to the gun. (If you don’t already own a capable compressor, expect to pay from $400 to $700 for an adequate model.) With this system, you can adjust the pressure and fluid viscosity as needed. The downside is that you have to filter the air to eliminate water, oil, and dirt from entering the gun and finish. When purchasing a compressor, consider a unit with wheels for moving it around the shop or outside. If you opt for a larger stationary unit, be sure to have plenty of hose.

Turbine–An HVLP turbine provides a fixed amount of air pressure and consistent fluid atomization in an efficient and dedicated delivery system. When shopping for a turbine spray system, you’ll encounter units with two, three, four, and five stages. This simply means the number of fan blades used to drive the air–the more blades, the more powerful the unit. Typically, air power in turbines is defined in psi (pounds of [air] pressure/square inch) and cfm (cubic feet/minute). Note that lesser priced units will have lower psi and cfm, and fewer stages–key differences. You always want to choose a turbine that matches up with the kinds of products you plan to spray. Lower-powered systems can certainly handle solvent-based products such as dyes, stains, lacquer, shellac, and oil-based clear finishes. But for thicker or higher viscosity products, such as latex paint, a more powerful turbine (or thinning) may be required. Note, too, that for water-based finishes, thinning is not an option, making application a challenge for some low-end HVLP turbines. These weaker units may produce a coarser coat when compared with the finer atomized coats of the same product laid down by units having more stages.

What Is Viscosity? 

The thickness of a finish product is referred to as its viscosity. Your spray system’s manual will help you determine the proper viscosity for the gun and needle/nozzle you are using and tell how to measure a fluid’s viscosity using a special cup. Generally, if the product goes through a medium mesh strainer at the same rate as you pour it in, it will spray efficiently. If too thick, the product will not atomize and may require thinning. When just right, the atomized product exits the gun as a fi ne mist, leaving a smooth, level fi lm on the workpiece surface.

The WoodRiver Pro HVLP Spray Gun (left) is a good low-cost option. It features a gravity-fed, top-mounted plastic pot, and adjustments for air, fluid, and pattern. Spend more on Apollo’s 7500QC spray gun for quality parts, fine adjustments for maximum control, and superior results. Both require a compressor large enough to spray a wide range of products.

Spray Guns: 

Siphon Guns–These old-style guns (not shown) rely on a jet of high-pressure compressor air to pull or siphon fluid up the feeding tube and out of an under-mounted pot on its way to the nozzle where it is atomized. Unfortunately, spraying with more air creates more overspray, less control, and greater waste. Enter HVLP guns. HVLP Conversion and Turbine Guns–Conversion spray guns connect to a compressor via a standard hose and have been modified to redirect some of the air to the fluid pot via a plastic air tube, thereby pressurizing it. With the gun’s trigger squeezed, the pressure forces the fluid into the gun and main airstream. At the nozzle the fluid is atomized into a fine spray and dispersed. Requiring less air pressure to operate, such guns offer better control and reduced overspray. Included here are low-end guns that have a gravity-feed fluid pot mounted on top. Also requiring less air to operate, these guns cost as little as $40 and can perform well right out of the box while handling a wide selection of finish products. What distinguishes higher-end guns from the other so-called HVLP guns comes down to quality, long-lasting parts and the ability to achieve a finer coat and reduced overspray. While some guns can work with either compressed air or a turbine, others are designed to work only with turbines and hoses with which they are sold. Another distinction lies in guns that are either bleeders or non-bleeders. With bleeders, air continually exits the nozzle, regardless of whether the trigger is pulled, a feature found in lower-priced guns. Non-bleeders contain the pressurized air until you squeeze the trigger. Related to this are the adjustments for dialing down the amount of fluid you wish to spray. These appear at different locations, depending on the gun .
Needle/Nozzles–A needle/nozzle combination works to atomize and disperse the product fluid in an atomized spray. With a smaller needle/nozzle, you get a finer spray. A larger needle/nozzle lets more material pass, but not as much of it may be finely atomized. This can result in an orange-peel or coarse coating. Needle/nozzles in the 1.3 to 1.5 range work best for most furniture finishes, but not for heavy-bodied latex paints. For these you’ll want 2.0 to 3.0. needle/nozzles. Buy additional needle/nozzles beyond those that come with the gun (from .8mm to 2.5mm) to handle all the finish products you plan to use. Beyond the needle/nozzles are the spray caps. These adjust to spray in a vertical fan, horizontal fan, a cone shape, and in narrow and wide swaths. Different guns offer slightly different adjustments.
Spray-Finishing

The needle/nozzles in spray guns come in a variety of sizes that determine the degree of atomization for a finish product.

Water-Based Update 

Over the last decade, water-based products have come into their own because of improved performance and environmentally friendly ingredients. Indeed, a movement is afoot worldwide to ban finish products with harmful materials. For instance, oil-based finishes are no longer available in California and in Europe. For safety reasons or because of where you live, you may want a system that effectively handles water-based finishes.

Spray-Finishing

This Badger 350 Deluxe Airbrush Kit is an excellent, low-cost choice for spraying small projects such as turnings and boxes. Requiring only a pancake compressor, its siphon-style gun lets you spray an array of thinned solvent-based finishes and paints.

Pots:

Three types of product containers exist: plastic, steel, and Teflon-coated. With plastic, water-based finish can build up on the inside walls of the pot, making it hard to clean. Teflon-coated pots clean the easiest. Another option is to use disposable liners to avoid the pot-cleaning chore.

Hoses:

While spray guns connected to a compressor tend to have the longest hoses, anywhere from 20' to 100', HVLP turbines have shorter hoses, measuring anywhere from a limiting 9' to 24'. The more powerful turbines can support a longer hose. Shorter, bulkier air hoses offer more challenges when spraying large projects. That said, you can easily relocate the turbine to where you need it.

Spray System Matchmaker

To help you find the spray gun or system suited to your needs, I created this chart relating the models showcased here to the type and amount of woodworking you may do. Use it as a guide but note that there are other products in the brand lines and other brand lines than those listed.

About Our Author

Charles Neil is an accomplished furnituremaker, finisher, and woodworking instructor residing in New Market, Virginia. Check out his Web site at www.cn-woodworking.com for more on his career, which has spanned over 30 years.

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