Spray Finishing Tips
Until I tested high-volume, low-pressure HVLP sprayers for a magazine article back in 1995, I'd always used a brush to apply a finish to my woodworking projects. After the tests, I was so sold on the new systems that I bought one. And it wasn't long before it came in handy. I had to finish 500 running feet of 6-inch-wide baseboard before installing it in my house, and I knew from experience that each coat of urethane would take most of a day to apply by brush. Using the sprayer, I was able to apply a flawless finish to the whole batch in less than two hours-no brush streaks, no stray bristles, and a consistent finish throughout.
If you're like most DIYers, you probably haven't had much experience applying spray finishes to wood. This is simply because, until recently, you didn't have access to spray equipment that was practical for your shop. Spray finishing in the home workshop is now convenient for two significant reasons: The new breed of reasonably-priced HVLP sprayers produce far less over-spray than traditional compressor-driven equipment, making it easy to set up a spray area in any small shop. Also critical to
Spray finishing is an entirely different process than working with a brush. Be prepared to make mistakes the first time you point the gun at a piece of wood. Practice on scraps before you spray anything valuable.
It's very easy to over-apply finishes with a sprayer, leaving ugly runs on your projects. Always be on the lookout for over-spraying, especially around outside corners that might get a double shot of finish, first from one direction, then another. The following, tips are mostly aimed at reducing the tendency of over-application and will help you improve the quality of the finish.
Keep the Spray Gun Moving:
Always release the spray trigger at the end of each pass, then press it again only after your arm starts moving back. Because there is a constant stream of finishing material flowing out of the nozzle as the trigger is pulled, you need to keep the spray gun moving. This is easy when you're in mid-stroke and spraying along an expanse of wood, but trickier when you arrive at the end of your arm stroke and must stop and reverse. That pause is long enough to cause runs if you keep the trigger pulled
Spray Only Horizontal Surfaces Whenever Possible:
Even if it seems to slow you down, spray items as they lie flat. This applies to both brush and spray applications, but is especially important when you're using a sprayer, because the chances of a running blob of finish are so much higher. Two-inch-long finishing nails hammered every 6 through 3/4-thick wooden strips make a great support base for finishing.
It also pays to pre-finish parts before assembly, especially the inside surfaces of cabinets, which are about as much fun to spray as the inside of your mouth. And finish items in several stages so, whenever it's possible, wet surfaces can rest horizontally until dry.
Use Light, Multiple Coats:
Apply each coat just a little heavier than a mist. Multiple thin coats are much better than a few thick ones. When you're watching that spray stream fan out from the tip of your gun, coating your project in a wet, glistening skin, there's a powerful temptation to lay on really thick. Don't do it. Not only will you have runs on your hands, but a thick finish won't dry well and could even turn into a bubbly mess.
Lighting is Crucial:
Use a powerful portable light (mine is a 500-watt quartz-halogen spotlight mounted on a plywood toolbox) to illuminate your work from a point just a bit higher than the workpiece. Ordinary overhead lighting just doesn't cut it. Be prepared to move the light as needed during spraying.
Spray finishing without an angled beam of light bouncing off the workpiece is almost as bad as spraying in the dark. Angled light is important because it allows you to see exactly where you've sprayed, where you haven't, where you're in danger of over-application, and where a run has actually occurred.
Always Carry a Brush
Chances are good that, despite your best efforts, runs will develop on your work now and then. I keep a 2-inch wide brush handy whenever I'm spraying so I can work these runs out as they occur. Runs never happen instantly, but usually develop several minutes after application. Check newly sprayed work about one minute after application and watch for trouble. The sooner you brush out runs the better. But don't try to brush out a run that has started to dry, since this will leave indelible brush strokes.