Woodworker's Journal 2011-35-Winter, страница 65
nicely through a large 0.070" (1.75 mm) tip, but I find many water-based finishes and thin varnishes work better through a smaller .040" (1 mm) or .055" (1.3 mm) tip. Better manufacturers offer a range of fluid tip, needle and air cap sizes to make the gun more versatile. If you plan to spray acid catalyzed lacquer, conversion varnish or water-based coatings, make sure all the fluid passages, including the tip and nozzle, are stainless steel. Regular steel will rust. This is standard fare on high quality guns. Each type of gun has its strengths and weaknesses, and which is right for you depends on your needs and how you plan to use it. No matter what you choose, there is no guarantee that high prices mean good quality — but for the most part, you get what you pay for. A good gun, even though it costs more up front, will be well worth it. How to Spray Before you spray anything onto your project, check out how your gun is working. Start with nothing but solvent in the cup, and spray some out onto a large piece of cardboard. Use the cleanup solvent for what you intend to spray, or simply use clean water. (If you use water before anything except water-based finish, you'll have to "chase" the water afterward by spraying some denatured alcohol.) With the adjustments wide open, the pattern should be oval and uniform. If it is not, it means the fluid tip needs to be cleaned (see Drawing at left). Turn the small adjustment knob from oval to round and back again while spraying to see the different patterns it makes. Set it to the pattern you want. A round pattern, which will deliver more material to a single spot, is great for getting into inside corners and spraying narrow stretchers Break down your gun by removing the air cap, fluid tip and needle. Soak them in solvent until they are clean. and chair parts. For spraying a panel, a long oval is better. Practice pulling the trigger more or less as you move the gun. The more you pull it, the more liquid will emerge. Set the gun to a vertical pattern if you want to move the gun from side to side. Use a horizontal pattern when moving up and down. Change from horizontal to vertical by rotating the air cap 90 degrees. Move the gun evenly in a straight line and always keep the tip about 8" from the surface of the wood. Resist the impulse to swing the gun up at the end of the board. Overlap each pass about one half the width of the last pass. Now empty out the thinner and fill it with what you intend to spray. Most materials designed for spraying should work without thinning. If yours needs to be thinned, it will form an "orange peel" pattern. Spray water-based coatings and oil varnish lightly so that the surface is just barely wet. Solvent-based lacquer can be sprayed heavier, until the surface is glossy wet. Practice will tell you a lot. Too wet and it will run and sag. (Beginning sprayers almost NEVER spray too dry.) Learning to spray is not any more difficult than learning to use a brush, but both take at least a little bit of practice. Cleaning the Gun When you are done spraying, open the cup and empty out the remaining finish. Pour in about a cup of the cleanup solvent for the finish and spray it through the gun to clear out anything left in the fluid passages. Break down the gun by removing the air cap, fluid tip and needle and soak them in solvent until they are clean. Remove the cup and clean it separately by rinsing and wiping it out. Scrub out the fluid passages with a pipe cleaner dipped in solvent. When all the parts are clean, put a dab of petroleum jelly on any threaded parts and put the gun back together. The needle gasket nut should be snugged up only hand tight. I like to add a drop of machine oil to the needle packing now and then. g / How Big? Any compressor will produce enough pressure (measured in pounds per square inch) to run any gun. However, the size and efficiency of the compressor determines how much air volume (measured in cubic feet per minute) it will produce. Standard guns typically need 2 to 5 cfin at 35 psi: even a lhp compressor is likely to be adequate. HVLP conversion guns may require from 7 cfin to 18 cfm at 60 to 80 psi and need a larger compressor. I've found that the gun tech sheets often understate their needs, while compressor plates often overstate output abilities. Therefore, buy a compressor that seems larger than what you need. Both portable and stationary air compressors can be suitable for spraying, provided they can produce the required air volume for the gun. WINTER 2011 65 |
