How to Grain Fill a Classical Guitar
By Orville and Robert Milburn
Pumice is of volcanic origin and has been around for hundreds of years and used for a variety of purposes. Before modern technology developed the fine sandpapers that are common in the marketplace today, pumice along with many other materials were used as abrasives. The wood finisher used a felt block and often a leather-covered block with pumice powder to sand the wood smooth. Pumice was also used to polish shellac and resins to fine high-gloss finishes. The pumice for the wood finisher was ground and filtered into many different grades and sizes much the same way as we buy different grades of sandpaper today. No doubt, at some time during sanding, it was discovered that the pumice not only acted as an abrasive but filled the wood grain at the same time. The pumice method of grain filling has been with us for hundreds of years and is still the preferred method of grain filling for many wood finishers.
Lets start our grain filling now . . .
After sealing, the classical guitar is allowed to "gas out" (remaining alcohol is allowed to evaporate) for a few hours. We are then ready to fill the pores and grain. This is the pumicing operation. At this point we should mention that there are some species of wood that do not require grain and pore filling. The top of the guitar will not have to be grain filled. Spruce, Cedar, and Maple are examples of "closed-pore" woods which do not have to be grain filled, and with some experience you will recognize woods which require grain filling and those that do not. Most rosewoods have large, open pores and will need to be filled. If the pores of an open-pored wood are not filled properly the finish will slowly shrink back into the pores. The result will be thousands of tiny craters in your finish.
Applying the Pumice
We will not be using any new shellac during the entire pumicing process. This is very important- additional shellac during the pore-filling process will inhibit the operation. We will be using the already dry shellac "spit coats" that we have previously applied to the guitar. All that is used is the muneca loaded with alcohol and a very small amount of pumice applied to the muneca surface.
The alcohol will dissolve the dry shellac while the abrasive action of the pumice is pulling off microscopic wood fibers which will be deposited into the pores of the wood along with the natural colored oils. Do not use any shellac while pumicing. As Eugene Clark says, "You would be better off if someone stole your shellac at this point." Your muneca must touch the surface of the wood. A heavy layer of shellac would prevent this.
|The muneca is touched to a bit of pumice on paper. The pumice is then distributed over the surface of the muneca with a finger.|
The actual pumicing procedure is simple and is as follows . . .
Add a generous amount of alcohol to the surface of the muneca. Be careful not to overdo the alcohol (about 10 to 12 drops is what we call generous). You will always have to add extra alcohol to a dry muneca. Once wet, however, just a few drops(4 or 5 for each recharge will be enough). Note that we will always load the muneca from the outside. Tap the muneca against the back of your hand to help distribute the alcohol throughout the wool pad. Fill your salt shaker with pumice and sprinkle a small amount onto a sheet of typing paper. Now, use the bottom edge of the muneca to "bite" off a small portion of the pumice. With the pumice sticking to the bottom of the muneca, add a few drops of alcohol (4 to 5) and rub with your finger until the pumice becomes transparent. This "clears" the pumice. NEVER APPLY PUMICE DIRECTLY TO THE SURFACE OF THE INSTRUMENT. Beware of clever gimmicks such as powder puffs and "pounce bags" used to add pumice directly to the surface of the instrument. They are unnecessary and silly.
Adding pumice directly to the wood surface will result in the particles of pumice showing through the finish. In addition, putting pumice directly on the instrument will result in piles of pumice that cannot be removed except by sanding. Any dry sanding at this stage may also cause the pumice to show through the finish. Now, press the muneca firmly to the guitar's surface and rub in small circular patterns. Do one small area at a time changing to a direction of rotation often. This is a good time to practice changing directions without stopping the muneca. Never use straight and forward pressure with the grain. This will tend to remove the material from the pores. Mentally divide the surface into small areas and finish these sections one at a time. Add only a little alcohol and pumice as you work. Soon you will see the grain being filled. You can actually feel and hear the muneca sanding the surface of the wood. When the muneca runs dry, recharge with alcohol and add a "bite" of pumice. Continue this same pattern over and over until you can no longer see the grain and pores of the wood. Be especially careful not to add too much pumice. You should be able to see any excess pumice on the surface of the guitar. It is now obvious why only a small section should be pumiced at a time.
|The pumice is made transparent by wetting it with a few drops of alcohol.|
Any excess pumice can be moved to an area of the guitar that needs the additional pumice. If you accidentally add too much pumice, add more alcohol and work the excess pumice to an area of the guitar that needs additional pumicing. The pumice is an abrasive and will tend to wear the muneca cover very rapidly. It will be necessary to provide a new cover periodically. When the pores on the back of the instrument are satisfactorily filled, turn the guitar on its side and repeat. Use only small circular motions changing from clockwise to counterclockwise movements as you work. When you encounter the heel joint, change to a small folded cloth and wet with alcohol. Add a bit of pumice and rub it into the cloth until it "clears." Rub the pumice onto the sides up to the joint until the grain has filled. This area of the guitar is perhaps the most difficult to pumice and may require 3 or 4 attempts before the pores are well filled. Next, you may want to pumice the neck of the guitar. Many luthiers would rather leave the neck natural without filling the grain. If you do choose to fill the grain, complete it exactly the same as if were the back or sides of the guitar. If the neck is Spanish Cedar or Mahogany be especially careful to "clear" the pumice as you work. Remember, don't use shellac as your pumice.
|The grain is filled by moving the pumice-loaded muneca in small circular motions. Direction should be changed often. The grain is best filled when the muneca is moved against the grain direction. Don't move the muncea in long strokes parallel to the grain.|
Now lets review the grain filling procedure . . .
- First, "seal-coat" or "wash-coat" all of the purflings with 3 sessions of shellac, being careful to protect the inlay work from color contamination. Seal the "field" areas similarly.
- While spit coating, fold your shellac cloth often to avoid discoloring the inlay work. Pull the cloth in one direction with a single motion. After spit coating the guitar three times, you can then paint more shellac on the purflings using a soft camel hair painter's brush. Do this if you are having contamination problems.
- Use no shellac or oil during the grain filling process. The spit coats of shellac with the alcohol, combined with pumice, will be all that is necessary to fill the grain.
- Remember to "clear" the pumice. Dab the wet muneca on a little sprinkle of pumice. Add additional alcohol and rub the pumice evenly on the bottom of the muneca to clear the pumice.
- Always load the muneca from the outside with alcohol. Be careful to not add too much alcohol. 10 to 12 drops to a dry muneca and 4 to 5 thereafter should be all that is necessary.
- Using firm pressure, rub the muneca onto the surface using circular motions and change directions often.
- Grain fill only small sections of the guitar at a time. Any excess pumice can be moved to an unfinished area using a little alcohol.
- Inspect your work often and use very little pumice as you work. Do not get impatient and try to hurry. This may result in more work