You can apply most types of clear finish to woodwork with a brush, but don't ignore the benefits of spraying. If you skip the brush, you can get a stroke-free coating in a fraction of the time, and you can get even coverage in crevices in wood that has turned or carved contours where a brush can't reach. Spraying allows you to layer coatings to get a truly glass-like surface, which makes a big difference when you're finishing tables, cabinets and countertops. Few homeowners have spray equipment, but that's not a problem. You can always rent what you need. You can also get top-notch results using an aerosol can, which is a great option if you have only only or two pieces to finish.
What Types of Finish Can You Spray?
Certain types of clear finish, such as nitrocellulose lacquer and two-part urethane, have to be sprayed, but you can also spray shellac, varnish and even penetrating oil. The only requirement is that the finish material be thin enough to atomize. Practically speaking, there aren't many good reasons to spray varnish, unless you buy it in an aerosol can. The fact that it dries slowly and can sag on vertical surfaces is a good reason to brush varnish on rather than spray it. But lacquer is a different matter.. Unless you're using a brushable material that contains a retarder, lacquer dries too quickly to brush. The same is arguably true for shellac, although many professional finishers do apply it with a brush or rag.
What Kind of Equipment Do I Need?
Professional finishers use air spray equipment, which consists of an HVLP spray gun, a compressor and an air hose to connect them. You can rent this equipment at rental outlets, but that's really only necessary if you're finishing multiple pieces of furniture. For a single piece, simply purchase the desired finish in aerosol cans. Turn the can into a spray gun by purchasing an inexpensive trigger assembly. It gives you more control of the spray and produces a wider spray pattern than the nozzle on the can. You can also apply clear finish with a handheld airless sprayer, but the high volume of material and the graininess of the spray it produces will require extra care on your part.
How to Apply Spray-On Clear Finish
There's no getting away from the fumes released into the air while spraying, so you need to protect yourself. Wear a respirator at all times and keep the space in which you're working well ventilated. Set up an exhaust fan if you're working indoors. The workspace must also be dry, because any moisture trapped underneath the spray will turn the finish cloudy. If you're spraying outside, do it on a dry day that has been preceded by two or three days of dry weather.
Things You'll Need
220-grit or finer sandpaper
Very fine steel wool
Step 1: Set Up Your Spray Operation
Find a good place to work and cover the floor and work table with newspapers. The best type of work table for spraying furniture is one that spins so you can spray all sides of the piece without changing position. Mask off any parts of your freshly sanded and stained woodwork that isn't getting finish. Set up one or two lights in the space to make sure you can see what you're doing. Turn on the fan.
Step 2: Prepare the Spray Equipment
Fill the cup of your gun, if you're using one, about 3/4 full of properly thinned material. Test the spray and modify the mixture as needed. If you're using aerosol cans, estimate how many you need for a single coat and have them ready so you can switch cans quickly.
Step 3: Apply the First Coat
Use sanding sealer for the first coat if you want to get professional results. Sanding sealer is a lacquer- or shellac-based product with a high concentration of soap-like additives that make it easier to sand. If a perfect finish is less important, you can also skip the sanding sealer and go right ahead and apply a coat of the finish material.
You can use both vertical and horizontal strokes when spraying with a gun, but when using a can, you may need to keep it upright, depending on the product—follow the label directions. This will limit you to predominantly vertical strokes. Don't arc during the stroke—keep the nozzle a uniform distance from the wood and pointed straight at it. Overlap the previous stroke by about half the width of the spray pattern on each stroke.
Step 4: Scuff With Sandpaper Before the Next Coat
Wait for the finish to dry before scuffing the surfaces with 220-grit or finer sandpaper. Lacquer and shellac dry tack-free in about 30 minutes, but varnish usually takes 2 to 3 hours or more. Scuff the finish until the surface of the wood feels smooth and you've leveled the grain.
Step 5: Apply Additional Coats
Wipe off the sanding dust with a damp tack cloth, then spray apply a second coat of finish, using the same procedure as the first coat. Repeat as many times as it takes to get the quality of finish you want. Most pros apply at least three coats for the smoothest possible finish.
Step 6: Buff the Final Coat
Rub down the last coat with a pad of very fine steel wool dipped in paste wax. Buff the wax with a soft, non-abrasive cloth. If you're using a compressor, you can also rent an air-powered buffer and buff with a lambswool buffing pad. Rubbing and buffing the final coat is optional, but it adds a depth of luster that turns a good finish into a great one.
- Clouding can be a persistent problem when spraying lacquer in humid weather. If the lacquer finish turns cloudy, don't panic. Do what you can to lower the humidity in the spray area, then apply another coat. The lacquer will re-emulsify and release the trapped moisture, and the cloudiness should disappear.
- When drips and sags occur on vertical surfaces, get rid of them by sanding after the finish dries and spraying over them. You may have to do this two or three times to make large drips disappear. Use the same technique to get rid of cracking or checking that occurs when the spray environment is too hot and the finish dries too quickly.
- If you use aerosol cans, you'll have to dispose of the empty ones. You can't just throw these in the trash if they still contain material. Make sure the cans are completely empty before recycling them.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.