Proper thinning
Finishes may require thinning to be suitable for use with a spray gun. The viscosity of a finish material can be measured with a special cup (viscosity cup) available from paint stores. Such cups are funnel-shaped containers that allow measuring the time liquid materials take to flow out of the container. Immerse the funnel fully into the material and start a stopwatch the very moment you pull out the funnel. For details of flow-out times consult the finish manufacturer's instructions.The ideal temperature for the material, the wood and the environment is room temperature.
Another far simpler method of determining the right viscosity is to watch the material run off a stirring stick back into a container: when it stops running off in one single stream and starts to break cleanly into drops, the viscosity is right. By adding solvents the viscosity can be altered. A lot of finishes, however, already come in thinned form and are all ready for spraying; with some others you have to add 10 per cent of thinner.
user the Wagner “FineCoat” HVLP system (see picture 8 on previous page), which is widely available, will do the job perfectly well.
The way you hold and move the gun during spraying determines the results that you will get. The ideal spraying distance is about 6 to 10 inches (15 to 25cm). If you hold the gun any closer than that, the particles will hit the surface with too much energy, resulting in an uneven surface. If, on the other hand, the gun is held further away, the material will dry before hitting the surface and will not flow very well; also, a lot of mist will be produced. HVLP spray guns ususally have to be held closer to the surface that is to be sprayed than conventional guns. Keep moving the gun evenly from one side to the other, each stroke overlapping the previous one by half. Avoid rocking the gun from side to side; instead, move it from one side to the other at a uniform distance and strictly parallel to the surface. Make sure that the material hits the surface at right angles at all times, even when spraying horizontal surfaces. If the gun is not held perpendicular to the surface, the spraying distance will not be the same in all areas, resulting in an uneven build.
The spray pattern can be varied by adjusting the amounts of air and fluid that are discharged; the thinner the material is, the less air is needed to atomize it. First close both the airand fluidadjustment knob all the way; the exact location of these screws varies from gun to gun and can be found out from the operating instructions. Then open the air-adjustment knob about onequarter and begin spraying. Continue spraying, while opening the fluid-adjustment knob, until you get a wet coat. The air-to- fluid ratio is right when the material begins to flow well on the surface. If you wish to increase the amount of fluid (finish material) that is sprayed on, increase the airflow and with it the amount of fluid that is discharged. Always test-spray on a piece of scrap cardboard first. The optimum balance depends on pressure, the viscosity of the material and the desired fan pattern.
Sanding the finish
After the first couple of coats have been applied use 320-grit sandpaper to sand the surface smooth and to remove all runs, sags and drips. Wet/dry sandpaper can be used. The advantage of wet-sanding is that this doesn't produce any dust. Use tepid water and add a bit of washing-up liquid to make sanding easier. Wipe off the sanded-off wet dust at regular intervals. On finished fingerboards the areas between the rets can be smoothed with a scraper.
Before the final coat is sprayed on, the guitar should be sanded particularly well. The final coat is best sprayed with a mixture made up of equal parts of thinner and nitrocellulose lacquer. This will give a surface that is easy to rub afterwards.
After applying the final coat hang the parts of the guitar in a dry, warm place and do not touch them for several weeks.
Several weeks later
No matter how well a surface has been finished - it won't look perfect until it has been buffed and polished. Polishing turns the surface into one single homogeneous area by reducing the size of the scratches on the surface, making them finer and finer until they are no longer visible to the human eye. However, before it can be polished a surface has to have hardened well: lacquer or water-based finishes take at least three weeks to fully cure as the solvents have to evaporate from all the coats of finish - and this just won't happen overnight. As long as the finish material can still be smelt in the drying room it has not yet fully cured.
In the picture on the left you can see me wet-sanding the body one haircut later, after about three weeks' of drying (of the body, not the hair!). Good lighting is essential for polishing. Before you start sanding, leave the paper (400-grit wet sandpaper) soaked in water overnight. When you do the sanding the next day, do so using water with a few drops of washing-up liquid in it. Wipe off the water at regular intervals and check the surface against some source of light by looking at it at an angle. Some areas will look flat, others, deeper spots which have so far escaped the sandpaper, will be shiny. To get a high-gloss finish all the shiny areas have to be sanded and thus removed. When this has been achieved and the whole surface has an even sheen, move on to 600and 800-grit paper. Sanding can be done in all directions, i.e. in swirls or in straight lines, as long as you do not rub through the coats. If this should, however, happen, a few more coats have to be applied.
1
Photo: Stewart-MacDonald's Guitar Shop Supply
3
4
5
2
Polishing the finish
To get a gloss finish you have two options: you can either move on to using a polishing compound, or you can continue sanding with 1000-, 1200and 2000-grit wet paper. If you don't want a gloss finish, you can use 0000 steel wool or synthetic wool to rub the finish to a low sheen.
Like toothpastes, polishing compounds contain tiny abrasive particles. They come in various degrees of fineness. Make sure to use silicone-free ones - otherwise problems might occur during refinishing. Since allmost all of these polishing compounds are normally used for polishing car paints, you should be able to get hold of them fairly easily from automobile body-shop suppliers. Although their staff will probably not know a lot about nitrocellulose lacquer, ask for advice none the less. I personally use the rather expensive “Finesselt” polishing compound by 3M. The ralley strip at the bottom of the bottle clearly shows what this product is intended for.
Ordinary polishing compounds can also be made at home by mixing water and rottenstone. The latter, which was once used for cleaning high-grade steel and silver, has now disappeared from our kitchens, but can still be found in paint stores.
If you want, you can do the polishing by hand only; however, using a machine will save a lot of time. Foam (1) or lamb's-wool polishing pads (2) which can be attached to a random orbit sander or a power drill (3) are commercially available. Put a lump of polishing compound on the pad and then spread it on the surface with the machine switched off. Then start the machine and keep polishing until the surface begins to mirror as the polish is broken down and turns into dry dust. Make sure that any excess polish is wiped off carefully before moving on to a finer grit; use a new polishing pad if possible, or wash the one previously used properly.
Finish polishing by hand. With this simple equipment and a bit of patience an excellent finish can be achieved (4). It may take a little longer, but as for quality it won't be second to surfaces finished with a buffing wheel. Some areas of the body such as the cutaways or parts of the neck can only be buffed by hand (5).
Metal parts, such as home-made bridges, are best sanded with wet paper: keep sanding in the same direction, using increasingly finer grits of paper until all fine scratches have gone. For finishing brass parts there is a special finish by the name of “Zapon”; nickel or anodized parts don't require any further finishing. They can be cleaned with chrome or metal polishes available from household goods stores.
Material needed for a nitrocellulose lacquer finish
Above you can see what is needed for finishing a surface with nitrocellulose lacquer: nitrocellulose thinner, sealer (which, as mentioned above, can be substituted with thinned lacquer), a can of nitrocellulose lacquer, polishing compound, rubbing pads, 400to 1200-grit wet/ dry sandpaper and a masking tape (from left to right).
Buffing wheels
Buffing pads make it possible to polish the finish particularly quickly and to access areas that would otherwise be difficult to get at. With a buffing wheel you can move from sanding with 600-grit paper straight on to buffing and get a high-sheen finish. Buying such a tool will, however, only pay off if you build guitars in large quantities – there is certainly no need for the amateur guitarbuilder to have one.
Polishing compounds also come in bars of various grades of fineness. These bars are rubbed onto the rotating pad. The object to be polished is carefully and lightly pressed against the pad from below and moved from one side to the other. Be careful, though: the high speed of the rotating wheel can easily cause the finish to overheat and lead to bubbles forming on the surface. With the wheel it is unfortunately also very easy to rub through all coats of the finish very quickly. Never even attempt to hold any of the body's edges against the wheel
– the force of it could send the guitar flying to the floor.
Before moving on to a finer grade of polish remove any remaining polish from the wheel by running it against the edge of a wooden stick. Better still, use two separate wheels, one for each polish.
The optimum speed is 700 to 1000 rotations per minute (rpm).
Photo: Stewart-MacDonald's Guitar Shop Supply