high when the gun is held 6 to 8 in. from the surface to be sprayed, as indicated in the photograph. External mix guns can usually be adjusted to produce a round 54&. External mixing cap. Fig. 5.5. Correct spraying technique. FURNITURE FINISHING 313 pattern for a concentration of liquid on small areas or modified by blocking off the side ports with friction tape. Held at a uniform distance from the work, the gun is swept across and back at an even rate to make one coat. During each sweep the gun nozzle must be held parallel to the sprayed surface; arcing will produce a disproportionately heavy deposit in the center of the stroke. In order to make the most of the fanshaped spray pattern, the gun should be triggered as the stroke is begun, ahead of the work, and released at the completion of the stroke, after the nozzle has passed beyond the surface being sprayed. A second coat applied in vertical laps will insure good coverage. Practice is the secret of good spraying, and can be exercised on scrap material or wrapping paper. Examination of test patterns before the finish coat is applied may result in necessary adjustments that will eliminate a later sanding off of an unsatisfactorily sprayed surface. Figure 5.5 illustrated the correct fan pattern, fairly regular in shape, with the atomization producing fine, evensized dots of paint across the pattern. In Figure 5.6 four incorrect patterns are shown. In detail A the small dark center quickly fades into rough gobs of material, indicating a failure in atomization which, if aggravated, will , , , „ , „ Fig. 5.6. Spray patterns. A, Heavy almost surely produce orange peel. Cor center. Bj Peanut. C) Heavy end. rection consists of either thinning the liquid D, Split spray. or increasing the airpressure. The shapes indicated in В and С are common distortions caused by a dirty gun. The split pattern in D is caused by excessive air pressure. Common Failures. "Piling" of the sprayed finish is caused by moving the gun too slowly or too near the surface. "Running" results from too thin a liquid. "Sags" are the result of too thick a liquid, too close a stroke, or improper triggering. "Pin holes" often occur when the gun is held too close to the surface, or from too much air pressure. "Misting" is due to excessive air pressure, a too thin fluid, arcing the gun, or holding it too far from the surface. "Floating" is usually the result of a poor mixture. Viscosity. As noted above, it is evident that in addition to manual ability to control properly the sweep of the gun, the liquid being sprayed must be of the correct body or viscosity. Many craftsmen are satisfied with a crude paddle test, during which they dip a thin strip of metal into the liquid up to a predetermined mark. When the "paddle" is withdraw the behavior of the liquid reflects its FURNITURE MAKING AND CABINET WORK relative viscosity. An excessively thin mixture will immediately run off the end of the paddle, whereas too heavy a liquid will sag down in drops before it runs.