Spray finishing

When working indoors with highly-flammable lacquers certain safety precautions obviously need to be taken. This is a major disadvantage of spraying. Water-based finishes do not produce any of the potentially explosive fumes and clouds of dust. A wellventilated room and a spraybooth for removing fumes from the air are needed. Any lights and ventilators in the spraying area must be explosion-proof as the cloud of fumes is highly flammable. If you need extra lighting, this is best provided by two normal lamps mounted to the right and left of you, and at some distance behind your back. The picture on the right shows one possibility of building a spraybooth around a window in a workshop or some other suitable room. The finish is best sprayed on in a room free from any sanding dust. If you do not have access to such luxury as a separate room for spraying, make sure that any other room used is properly cleaned before you start spraying (2).

I built my own spraybooth around a window by mounting a fan on a plywood board, masking both casements with cardboard and covering the top of the booth with another piece of cardboard (3). Just before I start spraying I spray a bit of water over the floor to bind any dust particles that might be there. The table is fitted with a small turntable.

Spraying has to be done in a low-humidity (not higher than 70 per cent) environment and ideally at room temperature or warmer. If the temperature is too low, the room should be heated. All potentially spark-producing appliances such as electric fan-heaters with thermostat must be switched off during spraying (danger of explosion!).

Out-of-doors spraying is of course also possible; do so only in warm, dry weather, though. No particular safety precautions are required for this, and you won't even need a spraybooth, although it will then be more difficult to keep the surrounding air clean. Another problem might be insects ending up on your guitar after being attracted and killed by the fumes of the finish material.

Using spray cans

Using spray cans is a cheap alternative to using expensive, complex color spraying systems: the cans are readily available, the propellant gases destroying the ozone layer are no longer a problem nowadays as they have been replaced by less harmful ones, and after use the empty cans can be disposed of as toxic waste so that no cleaning is required, either. Systems that allow using separate propellant gas cans are even more versatile: you can fill the cup with any finishing material yourself or custommix any color. With such systems you will, however, have to clean

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Explosion-proof fans

Explosion-proof fans have special aluminium rotor blades which cannot give off sparks when getting in contact with dust particles contained in the finish fumes. The entire motor is sufficiently well protected against overspray. Unfortunately, these special fans are very expensive and difficult to get hold of. A good and cheap alternative would be to have an external motor and a long V-belt for driving the fan propeller and to make sure that the motor is at some distance from the room in which the spraying is done.

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Spray cans

The picture above shows the cans needed for finishing. Can lids are easy to remove with a screwdriver. The can on the left (1) is a grey primer for finishing the body; next to it there is a can of blue paint (2) in a quantity big enough for spraying all the coats of the body. Using a primer improves adhesion of the finish to the wood and makes it easier to sand by filling uneven areas much better.

One can of nitrocellulose lacquer (3) is sufficient for spraying one neck.

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the jar as well as the nozzle very properly after use. Small, cheap spraying systems or airbrush equipment can also be used very effectively.

Shake the can well before use until you can hear the small steel mixing-balls inside it that help to mix the color. If you then continue shaking for another two minutes, the material should be mixed well enough to be ready for spraying. Using cans also has its disadvantages, though: it is difficult to control the amount of material that is sprayed on; there are no in-betweens: the can will either spray or it won't; a distance of at least 30cm (1foot) should be kept to avoid applying too much material in too short a time; and also the spray pattern (width and shape) cannot be adjusted.

When you start spraying never point at the body of the guitar as the can might at the beginning “spit out” larger lumps of color, which are not really needed on the body. Always start spraying close to, but off, the body and then move the can over the wood. I usually start with the sides. To do this I place the body on a turntable (4) and then hold the can at a uniform distance while turning the whole assembly in a full circle (360 degrees). Before pressing the trigger I point slightly away from the body with the can and then spray into the air for a short while. When turning the table the can has to be moved closer to the body in the cutaway and waist areas and further away again in the other, wider areas to ensure an even finish. As mentioned above, a minimum distance of about 30cm (1foot) has to be kept at all times. Resist the temptation of applying too much material: this would only make it run off like water and would produce drips and runs which would require a lot of sanding later that could be avoided.

For spraying the body top and bottom a handle provisionally mounted on the body is useful (5). Spray in strips from one side to the other and beyond the edge of the body, and then follow the same line back again. Ideally, a new stroke sprayed ought to overlap the previous one by half. For best results, hold the spray gun perpendicular to the surface and at a uniform distance from it; make sure to keep enough distance, and don't rock the gun from side to side. With this in mind, work your way from the top to the bottom. To reduce the risk of the can “spitting out” material spray everything in one pass; don't stop and start again, but keep going even if a bit of material may get lost at the edges – these are insignificant amounts which should not worry you. The first coat doesn't need to completely hide the wood texture. Spraying several thin coats instead gives far better results and is much safer.

After you have sprayed three coats, stop and allow to dry for half an hour.

A handle can also be mounted on the neck. Spray on three layers (6); in my case each layer consists of two thin coats, one sprayed on right after the other. After each layer wait for half an hour before spraying on another. After three layers (i.e. three times two coats) have been applied, turn the can upside down and continue spraying until only propellant gas is released. While you wait between layers, it is not necessary to clean the nozzle (by turning the can upside down). You should, however, always cover the can with the lid while a layer is drying.

Always wait until the finish has dried completely before sanding off any drips, drops, runs or other defects; wiping off a wet finish only makes things worse. If the finish looks like orange peel, the can was held too close to the surface and too much material was sprayed on so that it could not flow properly.

Using a spray gun

For conventional color spraying a compressor capable of generating a constant pressure of 25 to 80 psi is required. A constant pressure is easier to maintain with larger compressors. When using a spray gun the finish material is sprayed on at a high pressure, with a lot of mist being produced and at a great loss of material (20-30% efficiency only). Spray guns can be priced very differently, but the more expensive models (7) are certainly worth their money. Cheaper guns do not allow a sufficiently precise fan pattern adjustment so that a professio- nal-looking surface can't be achieved, at least not when applying color coats. Clear nitrocellulose lacquer gives quite good results even with cheap equipment.

HVLP spray systems (8) are a good alternative to spray-finishing with a compressor. Here the pressure required to atomize the finish material is generated by a turbine sounding like a noisy vacuum cleaner. When spraying with HVLP systems the color fan is wrapped into a cone of air so that less overspray is created and the finish is laid onto the surface very softly. They are therefore more efficient (65-90% of the material ends up on the surface) and allow for cleaner work. Although with such systems you can spray less material in a given time than with compressorsupplied guns, this drawback only really matters when working with large surfaces. Another disadvantage of HVLP systems is the fact that air comes blowing out of the gun continuously, even when the trigger is released; this can raise a lot of dust. An HVLP gun can, however, also be supplied with air by a large, powerful, expensive compressor. In this case the high-pressure air must be sent through a regulator transforming high pressure into high volume. Turbine-supplied HVLP systems are ideal for occasional spraying jobs as required by guitarbuilders. If you don't already have a compressor, get yourself an HVLP system. For the average

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Storing the nozzle parts

A clean nozzle is extremely important if the spray gun is to function properly. After cleaning all the removable parts of the spray nozzle I store them in lacquer thinner in a screw-top glass jar. Don't store gaskets in lacquer thinner as they become porous.