Spraying is a very efficient way to apply a finish. You can complete the job in a fraction of the time needed with other methods and the quality is usually hard to beat. The obvious drawbacks are the equipment expense and the need for a suitable space for spraying volatile finishes. Not many of us have a dedicated spray booth. However with the introduction of lower-cost HVLP (High Volume Low Pressure) spraying systems and user-friendly water-based finishes, spray finishing is now much more accessible.
THE RIGHT COMBINATION. HVLP systems are designed to minimize the amount of finish dispersed into the air as overspray and bounceback. Due to the lower air pressure needed, most of the finish lands on the surface of your project and stays put. Combine this with the fact that waterbased finishes are nearly odorless, non-flammable, dry quickly, and are less toxic, and you have a very desirable combination for use in small-shop finishing.
THE EQUIPMENT. There are two types of HVLP systems (photos above). An integrated system uses a turbine to provide a large volume of air at low pressure (about 10 lbs.). You use a large-diameter air hose and a gun compatible with the HVLP turbine.
A second option is to use an HVLP conversion gun along with a standard air compressor. This type works at a slightly higher pressure - 25 to 30 lbs., but you'll get equally good results. A conversion gun looks much like a standard, gravity feed spray gun with the same controls. If you already own an air compressor, this is a good way to go.
BASIC SAFETY. Although waterbased finishes are fairly benign, you'll still want to take some basic precautions. Your spray area should be well ventilated and free from dust. And then always wear a respirator.
STIR, STRAIN, & THIN. The three photos above show how to prepare the finish for spraying. Start by stirring it to an even consistency. Next comes straining. This will prevent small clumps of finish from clogging the gun or spoiling the film.
Finally, I like to thin my finish slightly. Water-based finishes have a high solid content and are relatively thick compared to other finishes. So to get better atomization, it helps to add a little water. Most water-based finishes can be thinned up to 20% without affecting the curing characteristics.
RAISED GRAIN? Water-based finish will raise the grain of the wood. There are several strategies for dealing with this problem. One is to pre-raise the grain by dampening the wood. Then you can sand it smooth prior to finishing.
A different tack is to use a sealer such as shellac. I like this approach because it has the added advantage of imparting a pleasant color to the wood. Finally, you can simply allow the first coat of finish to act as a sanding sealer. Spray on a light coat, let it raise the grain, then sand it smooth.
Now you're about ready to start spraying your project. I'll just offer a few basic tips to help get you off on the right foot.
ADJUSTMENTS. First, you need to adjust the spray pattern. A gun will spray a "fan" perpendicular to the fins on the air cap. The cap can be rotated to any angle depending on the orientation of the surface being sprayed.
The upper drawing above, shows the ideal fan pattern for most surfaces — an oval shape with a length that matches the optimum spraying distance of 6" to 8". A longer spray pattern will produce coverage that's too light, a condensed pattern too heavy. You can test the spray pattern on a piece cardboard.
LIGHT COATS. You'll get better results with several thin coats rather than one thick coat. You want to spray on a wet coat, but not one that looks thick and puddled. You shouldn't see a milky opaqueness to the film. Too little finish will leave a surface that appears dull and rough.
Don't worry if the wet film develops a slight "orange peel" texture. It will disappear as the finish dries.
HORIZONTAL. When spraying a horizontal surface, start at the far edge and work back toward yourself. Use back and forth passes, overlapping each stroke about 1/3 of the fan width. For consistent coverage, always keep the gun perpendicular to the surface, as shown in detail 'a' in the lower drawing above. As I mentioned, the tip should be about 6" to 8" from the surface. Start spraying beyond the edge and move slowly and steadily across the surface, releasing the trigger only when you've passed over the opposite edge.
VERTICAL. Water-based finishes have a tendency to run on vertical surfaces. The solution is to spray lighter coats. You do this by moving the gun at a faster pace.
RESPIRATOR. You should always wear a respirator when spraying any type of finish.
FINAL TIPS. I like to spray from the inside out. Inside surfaces are easier to access before the exposed areas are sprayed. And if needed, the project can be tipped or turned to get at hard-to-reach spots. When spraying in tight areas, trigger on and off quickly and try to keep the gun moving.
Spray along narrow surfaces like table and chair legs, not across them. Square edges can be sprayed diagonally along their length so that both adjacent edges are coated simultaneously. This helps prevent thick buildup of finish along the edge.
Once you're accustomed to spraying water-based finish, you might not want to go back to your past finishing ways. The ease, speed, and quality make it an awfully appealing option.