Spray Finishing Made Simple
You can't beat a sprayed finish for a smooth, even surface to enhance and protect a project. No worries about brush streaks or stray bristles embedded in the finish. Although spray finishing—done incorrectly—can result in drips or unevenness, with the right tools and finish, and a little practice, you'll soon be spraying like a pro.
Compressed air or HVLP?
A conventional compressor-fed spray gun operates at 40–60 pounds per square inch (psi) of air pressure, and does a great job atomizing finish and depositing a smooth, even film on a project. But that much air pressure causes blowback and produces clouds of overspray, lowering the quality of your finish and wasting about half of the finish. All that overspray must be contained, filtered, and exhausted from your work area. Air from a compressor inevitably contains water vapor and, for oil-lubricated compressors, fine drops of oil that can contaminate your finish. To prevent this, you'll need an inline filter or drip leg (shown above) located far enough downstream from the compressor to allow the water vapor and oil drops to cool and condense so they don't reach the spray gun.
That's why for small-shop spray finishing we recommend a high-volume, low-pressure (HVLP) system, shown above. An HVLP spray gun produces a more focused atomized mist, greatly reducing blowback and overspray, and depositing 60–80 percent of the finish on the project.
HVLP spray guns come in two varieties: turbine systems and conversion guns. A turbine system includes the gun, hose, and a turbine that generates dry, filtered air for the gun at 10 psi or less. The turbine output matches the requirements of the gun to ensure optimal atomization.
If you already have an air compressor and don't want to invest in a turbine system, a conversion gun may seem an economical solution—but with a hitch. A conversion gun looks similar to an HVLP turbine gun, but works at a slightly lower air pressure (20–30 psi) than standard air-pressure sprayers (40–60 psi).
It also demands a large volume of air (5–12 cubic feet per minute [cfm]) well beyond the capacity of most home-shop compressors. So before going this route, check the gun's air requirements against your compressor's specifications. If you need to upgrade the compressor, doing so might cost more than buying an HVLP turbine system.
Spray user-friendly finishes
Water-borne clear finishes have come a long way since their introduction three decades ago, and we now prefer them to oil-based finishes, which require an organic-vapor respirator and a spray booth with an explosion-proof fan motor. Without them, spraying an oil-based finish can be dangerous to your health and pose an explosion hazard.
In a marketplace crowded with similar water-borne finishes, choosing one may prove daunting. (See the photo below for a few of our favorite finishes.) Don't be afraid to try out more than one product. When you find a finish that works for you, stick with it for best results.
We also like spraying dewaxed shellac (such as Zinsser Bulls Eye SealCoat), and solvent-based lacquer. Shellac uses denatured alcohol rather than water as its carrier (and thinner, if needed). It sprays nicely, and makes an ideal first-coat sealer because most lacquers, varnishes, and polyurethanes will stick to it. If you use it alone, or topcoat with regular shellac, you'll get a durable finish with a glossy sheen. Lacquer uses lacquer thinner as its base. It sprays beautifully, but has a strong odor. As with oil-based finishes, spray shellac and lacquer only in well ventilated areas using explosion-proof exhaust fans; these start at around $600.
How to measure finish viscosity
- Stir the finish well, but don't create bubbles. If you do, wait until the bubbles dissipate before you measure.
- Submerge the cup into the finish with the rim just below the surface and keep it level.
- Start the stopwatch at the exact time you lift the cup straight up.
- Let the fluid flow out until you see the first break in the stream, then stop the timer.
- The number of seconds equals the viscosity measurement. Cross-reference this against the chart in your gun's owner's manual to determine which nozzle to use for that finish, or thin the finish until its viscosity matches your nozzle's specifications.
Source: Ford #4 viscosity cup.
Finish consistency is key
Tip! Smartphones have a stopwatch you can use for timing fluid viscosity.Some water-borne topcoats require thinning 5–10 percent (1–11⁄2 oz. per quart) with distilled water for spray application. Establish a baseline measurement by checking the viscosity of the unthinned finish with a viscosity cup, shown above. This gives you a feel for how much water to add to any finish to make it sprayable. Start at low dilution and test for good material feed, gradually adding water until you obtain a consistent spray pattern and good coverage. Thin only the amount of finish you intend to spray in one session, and always thin your finish using the viscosity-cup time rather than the amount of water added in previous sessions.
Set up a spray space
Ideally, you'd have a dedicated spray booth to work in, but for most of us, that's unrealistic. Still, you can make a spraying setup in your shop or garage, even if a temporary one, for water-borne finishes. Start by placing a box fan in a window, with a pleated filter secured to the intake side (facing you). If possible, use two fans to maximize exhaust ventilation.
For small projects, place a work table in front of the window (shown below) and create a cardboard shroud to contain and funnel overspray and fumes to the fan.
For large projects, fashion a booth with white muslin drop cloths. Don't forget the ceiling: Dust and grit from overhead can land on a freshly finished project. Avoid using sheets of polyethylene, because static electricity can cause dust to cling to those sheets, and that dust may find its way onto your wet finish. The fan will draw air from beyond your spray space, over and around the project, and out the window, so clean the area outside the spray space, too.
Arrange good, bright lighting that reflects off project surfaces so you can see how the finish builds, below. Wear a respirator made to filter organic vapors. HVLP turbines can be as loud as a noisy shop vacuum, so wear hearing protection when using one. Don't place a turbine under a box to muffle the sound; without proper ventilation, the turbine motor will overheat.
Develop your technique
Before spraying a project, build your confidence by practicing on sheets of cardboard ( below). Adjust the finish volume, size and pattern of spray, the speed at which you move the gun, and the distance from the workpiece (6-8") until you get good, even coverage without drips. Cardboard absorbs finish differently than wood, so once you get the hang of spraying on cardboard, try veneered plywood or finish-sanded scrapwood.
* Seal coats. Water-borne finishes leave raised wood grain after drying. You can knock this down with fine sandpaper, or avoid it altogether by first spraying a seal coat of dewaxed shellac or lacquer. Start by finish-sanding to at least 180 grit, then remove all sanding dust with a clean rag or shop vacuum. Spray on one or two light coats, let the finish dry thoroughly, and then rub it out (buff smooth) with 320- or finer grit sandpaper. If you've stained the wood, spray two seal coats before rubbing out to avoid sanding through the stain. (A thinned finish builds less quickly than an unthinned one, requiring two seal coats and usually more than one finish coat.)
* Stains. Water-borne stains raise grain more than water-borne polyurethane, and can't be easily rubbed out after application. In this case, raise the grain before applying the stain. First, sand to 180 grit, then wet the wood slightly with a clean sponge. Wait for the surface to dry, and finish-sand again. Apply the stain and let it dry. If any grain raises again, wipe it gently with very fine abrasive, and blow away the dust.
Rather than going to this much effort, we recommend wiping on an oil-base stain and letting it dry at least 12 hours (until you can sniff the surface and not smell the solvent) before spraying with a water-borne topcoat.
* Topcoats. Spray on finish until it fully wets the wood and leaves a bit of stippling or orange-peel texture on the surface immediately after application. (It will soon level out.) Spraying until the wet surface is absolutely smooth means you've applied too much finish. If your finish has a milky color in the container, and you see this color building on the workpiece, once again, you've applied too much finish. In either case, quickly wipe off the finish with a wet rag, wipe the piece dry, and start over.
Spray finish with confidence
Hone your spray-finishing skills by practicing these tips until they become habits.
- Pull the sprayer's trigger before you reach the edge of the project, move across the project at an even pace, and release the trigger after spraying past the far edge. Starting the spray directly on the project can create uneven or blotchy finish, drips, or runs.
- Overlap the spray pattern about halfway onto the previously sprayed pass. Maintaining a wet edge in this manner helps you blend each swath.
- Keep the tip of the spray gun parallel to the surface as it moves across the project, and move it at a steady pace. Don't swing the gun in an arc.
- Apply just enough finish to a vertical surface to wet it without creating runs.
- Apply multiple thin coats rather than one or two thick coats.
- Move the spray gun across the surface at the same speed you would move a brush when "tipping off" a brushed-on finish.
More spraying tips
When you're ready to spray your first project, do the following:
- Spray as many parts as possible in a horizontal orientation to reduce the likelihood of drips and runs.
- When spraying any type of casework, always spray the interior elements first, then the outer parts last.
- Leaving the backs off cabinets and case pieces provides easy access to the interior, especially tight corners. This also prevents finish from blowing back in your face.
- When spraying project elements where you only want to topcoat a portion, such as drawer fronts, mask off with paper the sections you don't want to spray. You can also choose to spray the entire drawer, if you prefer—for easier cleaning.
- Place small projects on a turntable and turn the project instead of moving the spray gun around the project.
- With almost any finish you spray, sanding between coats knocks down any dust nibs, making for smoother succeeding coats. And with polyurethane, the sanding marks help the new coat link to the previous one.
- Don't stand between the project and the exhaust fan, or you'll block the fan or get coated in overspray.
To get a video lesson on spray finishing, visit woodmagazine.com/sprayfinish