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Spraying is an entirely new game with a different set of rules from bristle brushing. Except for fine detailing and possibly some weathering techniques, it will produce a far superior finish. Coats can be thinner to avoid obscuring detail and more even, especially on irregular surfaces. Masking is often easier and division lines are cleaner. But all this is not attainable without good techniques, quality equipment, a fair amount of practice and a place to spray. A tome could be written on the more advanced techniques, but the basic principles discussed can provide a foundation for developing these to your taste. Although alternatives are available, airbrushing can achieve the finest finishes. Prime requisites are a good airbrush, a non-pulsating air supply, pressure regulator, moisture trap, connecting hose and fittings, good paint, something to catch overspray and, if using toxins, a means to remove fumes. Acquisition of all necessary components may be quite expensive, but in the long term, may be the most economical. Understanding the properties of spray patterns and the effects of other variables is essential to attain desired results. Primarily in hobby sprayers, the mid-air pattern forms a right circular cone with apex at nozzle. Spread angle is determined by nozzle tip form, but may be effected by large pressure changes. A perpendicular aim produces a circular pattern on the receiving surface, while other angles yield ellipses. Of course, if patterns are larger than surface, they are truncated and the parts that miss, produce overspray. The size of the surface pattern reduces as the nozzle approaches it, which through, less spreading, produces a greater density of droplets in the mist, resulting in a thicker coating. With a good tip, droplet density will be constant across a plane at a fixed distance. Thus for an even coating, all passes must held parallel to and equidistant from the surface. A good way to achieve this is to glide the forearm along the straight edge of a bench or spray booth. Except for special techniques, the tip should never be swiveled around wrist or elbow, traveling in an arc. Contrary to bristle brushing, succeeding passes may be in any direction. Frequently, due to rapid pressure changes, nozzles spit, when flow is started or stopped. Complete pattern must be off the painted surface, when these events occur. When masking, to reduce build-up along and possible seepage under the shield, spray should be lightly angled off shield edge; never into it. NOTE: Excessive air flow velocity or poor air flow paths through booth and around object can distort spray pattern, thus reducing fine control. See SPRAY BOOTH EVALUATION for a thorough discussion. Since they are interactive, control of other factors is crucial to obtaining desired results. Distance, flow rate, pressure, humidity, tip size, dust, temperature, paint characteristics and mixtures are all critical. As distance decreases, layer thickness increases and often build-up appears, while fine detail disappears. In the opposite direction, the layer may thin enough to prevent coverage. Adjusting the flow rate will correct most problems of this type. Very often adjustment is done by trial on a scrap piece, prior to starting a technique change. Speed of the pass also effects coat thickness in that slower motion produces a thicker layer. Unless a special effect is desired, for base coats, speed should be constant throughout pass. A very rapid pass, with low flow rate and pressure, can produce a mottled or dust effect. Better quality airbrushes provide flow rate adjustments by varying nozzle aperture or changing tip size. Larger tips or openings increase flow rate producing a thicker layer. The cone angle normally increases with larger tips. Excessive flow rate my cause running or puddles around details, which may fill cavities. Since airbrush regulators tend to be expensive, pressure adjustment is too often ignored. The effects can be quite noticeable during spraying and very often in the finish. Recognition grows rapidly with experience through application. Particularly apparent with fast drying lacquers and solvent paints, drying often occurs during transit from tip to surface. For base coats, medium should arrive wet enough to ensure leveling, but not enough to cause runs. If, through longer distance, transit time is sufficient for paint to dry, a powdery finish will result. Increasing pressure will increase velocity and reduce transit time. Low pressure with high feed rate may produce sputtering or high pressure with short distance may produce spattering. Since inexpensive regulators do not usually yield readability and repeatability in the low pressure ranges required, investment in a good one (ie. Paasche R-50) is well worth the extra cost. Paint or coating selection criteria are quite different from bristle brushing, although the same media may be used in either with varying effectiveness. Since most paints are offered at a consistency suitable for bristle brushing, selection becomes more mystifying by their thinning qualities. New factors are: paint must siphon from a pool, pass through a restrictive nozzle, fly through the air, level from tiny raised dots rather than ridges and not run down surface. Although some may overlap, each introduces it own requirements. Viscosity of vehicle is probably the first noticed, since most media must be thinned for the fluidity required in spraying. The vehicle in which the pigments ride, minimally consists of thinner and binder or adhesive (glaze). Retardants or accelerants may be added to control drying rate. The thinner evaporates, while the binder adheres both the layer to the surface and binds the pigments together. Often in more modern coatings, it forms a very thin plastic sheet. Excessive thinning may reduce adhesion and opaqueness by spreading pigments plus accelerate drying. Large pigments may preclude sufficient thinning to siphon or pass through finer nozzles. Resulting from the finer layering, usually achieved, and thinning; opaqueness , through finer pigments, grows more demanding. Very often more than one coat is required to obtain desired coverage, through overlapping pigments. This may nullify some of the advantages of thinner base coats, but provide others in weathering. However, even with multiple coats, total layer is usually thinner than other methods. The time restraint between coats may be critical, since the addition of a succeeding coat to a still fluid undercoat may produce running. For this reason and possible heavier swath overlaps, attempts at complete coverage with one coat should be avoided. The usual procedure is to paint successive surfaces around object by rotation, eventually returning to the first for the next coat. Aggravated by thinning, drying time becomes more crucial. Often paint dries in mid-air before reaching surface, preventing sufficient wetness for leveling. In more extreme cases, the raised dots remain completely separated, producing an excellent powdery finish. Reducing the distance or increasing the flow rate or pressure can help, but frequently a retarder must be added to preserve the desired properties of these adjustments. Water based acrylics have a relatively slow drying time, which is lengthened by water thinning. Substitution of alcohol or compatible thinner reduces comparative time and improves fluidity. Required thinning also reduces adhesion by diluting bonding agent (glaze) to a point where addition is required. Bonding agents are often colored and may alter original hue. Floquil's Glaze has an amber tint. There are definite limits to thinning set by coverage, adhesion and drying requirements. Selection of good hobby paints and coatings is limited, since many types can not meet requirements. In general enamels can not produce thin, quality finishes and flow is poor. Dope and some epoxy paints can be sprayed effectively with larger nozzles, but coats are normally too thick. Acrylics are excellent sprayers , if thinned properly. Although the vehicle is normally water based, alcohol is preferable, since it reduces surface tension, improving flow and also accelerates drying. Poly S thinner is superior in their paints. Solvent based paints are usually the best sprayers, since vehicles are thinner. Although it produces a thicker coat, which is still thinner than bristle brushing, many experienced painters spray Floquil without thinning. However Scalecoat does require thinning. Both can be successfully sprayed on annealed styrene, by spraying so that coat is almost dry, to reduce penetration. The solvent dissolves a very shallow layer to which the paint fuses, forming a bond that strongly resists removal. Because containers often cost more than contents, buying thinners in 1 oz bottles is quite expensive. Quart or gallon cans lower per ounce price to a small fraction. Since many media dry rapidly, clean-up immediately after use or between color changes is mandatory. A couple of one or two ounce bottles of thinner should be kept handy to simplify job. Last used bottle adapter should be allowed to drain slightly and wiped clean with a paper towel. One bottle can be attached and thinner swished to rinse paint from around tube and lid. A short period of spraying thinner through brush will clean most media from inside tube. A second bottle of absolutely clean thinner is attached to adapter and sprayed to finish cleaning. Religious practice of this will often delay disassembled airbrush cleaning considerably. For easier transfer of thinner from cans to bottles, Perfect offers fuel pumps that fit and seal most cans. A plastic tube extension can be added to reach bottom of taller cans. Often the clear, translucent, glossy, matte or flat coatings in some lines of paint use different vehicles from pigmented paints. Some solvents are not completely compatible with decals, paints or plastics. Commonly used, acetone is a solvent for cyanocrylate cements. It can soften them to release glued fingers. Due to lack of pigments, viscosities are lower and most spray well with minimal thinning. Although often recommended, automobile and other lacquers and finishes do not possess the qualities required for fine modelrailroad finishes due to larger pigments. At normal distances and viscosities, different pressures are required to spray different types, brands and even colors in the same line. This is dependent partly on weight of vehicles and pigments. Most acrylics spray well at about 20 psi or less if non-water thinner is used. Thinned Scalecoat may require as high as 30 psi. With the most commonly used, Floquil, experienced sprayers often notice a wide spread among different colors. White sprays best at about 15 psi, while black does well at 8 or 10 psi. White gives the illusion that pigments float through the air, while black appears to spray as if it were all liquid. In other colors, there does not appear to be any direct correlation between brightness and pressure. For future reference, it is advisable to maintain a notebook to record successful combinations of dilutions, tips, pressures and other variables. It could be extended to include successful, paint color, mixing formulae. Feedback from customers indicates that the new formulation of Floquil, required by the EPA to protect me from me, has ruined the excellent qualities. Fortunately I have an adequate supply of "old dangerous stuff" that I have been using for about 50 years. BACK TO PAINTING AND COATING BACK TO SPRAYERS BACK TO METHODS INDEX |
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