How to Seal-coat a Classical Guitar

We are now ready to apply a base for the French polish. This base is what is known as a "spit-coat", "wash-coat", or "seal-coat" of shellac. The purpose is three-fold. First, it provides a bonding surface for the many microscopic layers of shellac that we will be applying to the classical guitar. Shellac is known as the worlds best wood sealer because of it's high bonding properties. Second, it will help protect the inlays, purflings, and the rosette from color contamination due to the oils and dyes in the back and sides of the classical guitar. Third, this base of shellac will combine with the pumice and natural wood dyes binding them into the pores of the wood.
Sealing the edges of the back of a rosewood guitar. You can easily see the seal coat left by the wet pad.
The first job will be to seal the purflings, bindings, and back strip. This will help to keep the oils and natural dyes in the wood from discoloring the inlay work. Cut an ample piece of t-shirt material into about a dozen 4" x 4" squares. Fold a 4" x 4" square of t-shirt material in half and then fold once more into quarters. With your squeeze bottle of mixed shellac, saturate a corner of the folded cloth pad until thoroughly wet, then apply a drop of oil to the same area. Let's do the back of the guitar first. Drag the cloth pad over the purfling and bindings following the contour of the guitar. Do only one half of the guitar at a time. This should be done in a single pass without stopping and with no side-to-side movements. If done quickly and accurately, no color will be left in the inlays. Move to the other half of the guitar and turn the cloth over to a clean side, recharge the cloth with shellac and oil and repeat.
Sealing the edges of an instrument. Note the "field" is already sealed. Here we are sealing the purflings and bindings a second time.
Having completed both halves of the guitar, re-fold your cloth to a clean quarter, recharge and seal the back strip in a single straight pass. Now re-fold your cloth pad to the last clean quarter and recharge. In circular motions, wipe the "field" (unfinished areas of the back). By now your cloth pad is well contaminated with color from the wood. Dispose of the cloth and make a new one identical to the first. With a clean cloth, wipe each side of the heel joint with a single pass. Lay the guitar on it's side and seal the bindings and side purflings exactly in the same way as we did the back of the guitar. When the purflings are covered, use the cloth to cover the fields. Take care to fold the cloth to a clean quarter after each pass. Discard the color contaminated cloth as you fill in the field areas. Repeat the entire process for the other side of the guitar. Note: discard the cloth anytime you are concerned about dragging color onto light wood and causing contamination. After completing the sides of the guitar, apply a seal-coat coat to the rosette and the top purflings, re-folding the cloth to a new quarter each time. With a clean cloth, coat the field areas of the top.
An example of a folded pad after it has been used to seal a section of a guitar edge. This coloration will contaminate white purflings and light wood.
You will have to make a new cloth each time you run out of clean quarters. We will repeat this process three times on the entire guitar. For each pass, we will re-fold our pad to a clean side. It's a good idea to always make a new folded cloth pad to do the top of the guitar to avoid color contamination. It will take only about 15 or 20 minuets to seal-coat the entire guitar. When the first seal-coat is on, take time to examine the purflings. If, by chance, some color was drawn into the purflings you may want to take a small rubber block and sand the contaminated area using olive oil and 400 wet or dry sand paper. If you are adept at using scrapers, just scrape the discolored area and re-coat with shellac.
Removing purfling contamination using a small scraper made from a heavy single-edge razor blade
After letting the first spit coat dry for about half an hour, repeat the process. Each time that you apply the shellac the cloth pad will stay a bit cleaner. This is an indication that the purflings are becoming well sealed and a good base of shellac is being laid down. After applying three spit coats to the entire guitar, we will take a small artist's brush and again paint all of the purflings to provide additional protection for all of the inlay work since pumicing acts as an abrasive and can burn through the shellac to the purflings. In addition to painting the purflings, you may wish to apply a wash coat to the classical guitar machine slots as well.