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Wood Preparation


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Wood preparation may be the single most important element in obtaining a professional mirror like final finish and also the phase of finishing often most casually approached. Raw wood doesn't always reveal it's secrets. In fact without careful inspection during the preparation stage many misdeeds will not be evident until the final clear coating.

Once the wood has been stripped inspect the wood to locate any problems which may become offensive. Search for such things as very deep sanding scratches, cross grain sanding or dents in the wood. Be especially diligent in searching the areas where cross grain sanding may have most likely occurred such as the area behind the bridge on an arch top or at the heel of the neck. Scratches which may be nearly invisible will become evident when dye is applied. The dye will stain the exposed end grain in the scratch and darken it. If you find a sanding scratch after the dye has been applied the dye (and the scratch) will have to be sanded out and restained.

Deep dents are most common on the back but don't overlook the peghead sides and ends and the area around the jack. If the dents are very deep they can be filled with a colored wood filler and sanded flat. A fine point graining pencil can be used to draw in grain lines if needed. Now is also the time to try and locate any low spots caused by careless (read blockless) sanding. Although barely perceptible now these swales will look like deep rounded valleys after the final polishing. Most sanding swales can be sanded flat with a sanding block. Deep swales can be cured with repeated fills of a clear nitrocellulose sand and sealer such as Parks.

One other problem should also be noted. Sometimes when chemically stripping the old finish will melt and act as a dye. Black paint is the worst followed by red. If the plan is to recolor with the same color as the color that was removed there should no problem. But if a different color or natural finish is desired and the old color still partially remains bleaching may be needed.

In my experience colors can be removed with two or three applications of chlorine laundry bleach. Some color may remain in the grain but usually not enough to affect the final finish. Water marks can be removed with oxalic acid crystals. Most hardware stores carry this type of bleach.

With the wood clean and sanded. The finish prep can be begun. The steps given may or may not be needed with a particular wood. The steps following will assume the wood being finished is a "worst case" wood, i.e.; mahogany. Rosewood, ash and walnut also fall into this category. (Note that "worst case" woods are not really that. The oily woods such as paduak are the real worst cases.)

The first step is to wipe down the wood with naphtha. This will remove any oils that may have gotten on the wood during handling. When dry the wood should be filled with a grain filler. My personal choice is an oil based grain filler. I am sure that water based fillers can work equally well. Grain fillers are available at all good wood working stores and some hardware stores. When working with a colored filler it is best to seal the wood with one or two coats of lacquer to prevent the stain in the filler from staining the "field" area of the wood. It is best to let the sealing lacquer dry overnight so that it dries into the grain and does not displace the filler.

The filler can be wiped on with a cloth although I favor using my fingers. When slightly dry (dry enough that wiping does not pull the filler from the grain) the excess can be wiped off with a rough cloth. If you are careful it can be gently scrapped off with a single edged razor. In either case wipe or scrape across the grain. The razor method seems to pack the excess filler into the grain and is why I prefer it. Remove as much as you can now while the filler is slightly wet and you will reduce the time needed in the later sanding step.

When dry (overnight) the wood can be sanded clean. Time can be saved before by using a rag damped with mineral spirits to remove the majority of excess grain filler. If you decide to wipe off the excess filler let the filler dry again for a few hours before sanding. Using #180 dry sand off all of the filler in the field of the wood. Be sure to get the filler that is in the neck to body joint, in the switch holes and over the edge and in the pickup and control cavities. Filler in the screw holes does not hurt and may even help in that the filler keeps water out of the holes when doing the final wet sanding. (Water in the screw holes may cause the wood to swell and result in lacquer chipping around the screw holes). When sanding always go with the grain of the wood to prevent scratches. Follow with #220 dry sanding.

After sanding clean I suggest (and religiously use) a second filler treatment to achieve an ultimately glass like finish. First the reasoning. The glass like finish of a nitrocellulose finished guitar comes from the multiple application of lacquer and removal by wet sanding. And although the desired finish can be obtained by multiple lacquer coating only, there are two problems which at their base are both time related. The first problem is that multiple coatings (something in the 20 to 30 coat range) and sanding flat after every few coats is in it's self very time consuming. And secondly nitrocellulose, while drying to touch and a practical use level very quickly, it continues to dry over months and perhaps even years. A finish that appears mirror flat when applied will continue to dry and if the grain was not filled totally flat, in a few weeks, the grain may begin to show as the lacquer continues to "dry in".

After the wood has been filled spray with one or two coats of lacquer to lock in the filler and color. To achieve a mirror finish relatively quickly (and one that will remain stable as the final lacquer coats dry) next apply a high solid content sand and sealer product. Parks Corporation makes a nitrocellulose product which fills this requirement nicely. The application is simple as the sealer can be brushed on. Brush on the first coat and allow to dry for a few minutes. When the surface is flat (non reflective) a second coat can be brushed on. Allow this double coat to dry at least an hour (I usually wait overnight). When dry the coats can be block sanded flat. I suggest that this sanding be done in an area where dust is not a concern. You will see why when you begin sanding. I also suggest that you wear a mask. When this two coat application has been sanded flat (no shiny spots) wipe down with a dry cloth, inspect again for shiny spots and repeat. That is; apply two more coats. Paint them on as before with time allowed between coats for the coats to dry flat. When dry (again at least an hour) sand the coats flat. During sanding wipe off the dust to aid in locating pits which will appear shiny. The base you are creating will be the foundation for the lacquer finishing coats. Unless corrected, any pits, runs or unfilled areas will show in the final lacquer coats. Because of the high solid content of the filler, corrections can be made much easier at this stage.

The base should appear flat with no pits showing. If you find pits they can be drop filled with the sand and sealer and sanded flat. You will find that pits and swales that are too deep to be safely sanded out can be made invisible with repeated fills. Color coats and clear coating can now be applied.

^ReRanch

Sorry for the long post though :D

Later,

Chris

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