Refinishing a Martin guitar, as with any guitar, is a difficult and time-consuming process. A Martin guitar is a high-quality instrument. A poorly done finish may look worse than leaving it alone. Before you take on the task, be sure that you are willing to put the time and energy into it. You'll need to find a place to lay out your work for several weeks while you complete the project
Remove all the hardware from the guitar. Tape over the fretboard and bridge completely. Lay the guitar face up on your workbench on top of several layers of newspaper.
Use a brush to spread stripper over the top and side of the headstock and over the body of the guitar, brushing top to bottom in the direction of the wood grain. Allow the guitar to set for 15 to 20 minutes.
Use the plastic scraper to gently remove the loose finish. Work the scraper in the direction of the wood grain. Wipe off the guitar body with a rag.
Repeat re-coating the guitar with stripper. Wait 7 to 10 minutes and remove any remaining finish with the scraper and rag. Repeat again until all the finish is removed.
Wet sand the guitar gently in the direction of the grain to remove any rough spots. Finish by wet sanding with ultrafine sandpapers (400- to 600-grit). Flip the guitar over and do the back side of the body and the headstock the same way until the finish is entirely removed. Lightly paint the guitar with acetone to neutralize any remaining wood stripper and allow to dry.
Wipe oil stain into the body of the guitar with a soft, clean, dry cloth. Wipe the stain off with a dry rag.
Sand discolored spots in the stain where the old top coat wasn't completely removed with dry fine grit sandpaper.
Apply stain with a cotton swab gently moistened with stain. Brush on a bit at a time until the spot matches the surrounding area. Repeat until the spots are blended into the finish. The shellac finish will help further disguise any discoloration, but do try to get as much of it as you can.
Apply a second coat of stain to blend the patches and darken the stain. If you wish a lighter stain, you may dilute the stain with paint thinner before application. Allow the stain to dry and cure overnight. Apply any decals you plan to add to the guitar.
Mix enough shellac to use for one coat on the guitar, following the instructions that come with the shellac flakes. Apply only high quality shellac without additives, as additives can interfere with the process of french polishing.
Rub a drop or two of olive oil on an area of the guitar. Press the shellac firmly into the area with a three-layered applicator consisting of a muneca, with layers of wool on top, and old T-shirt material wrapped around it. The oil helps the muneca spread the shellac evenly and it is pressed out by the firm application. Apply one layer. Replace the T-shirt cloth cover when it becomes discolored.
Apply two more “spit coats” until the T-shirt cover of the muneca remains mostly unstained. Each coat of shellac blends with the previous coat to create a deep clear coating. Allow the shellac to set for half an hour between layers. Allow to cure, and then apply shellac with a fine brush to any exposed fitting corner, edge or purling that could be damaged by the polishing process.
Make another clean muneca applicator with a layer of muneca, three layers of wool and a wrap of T-shirt material. Load it with a light amount of alcohol and pumice polishing compound from the top. Rub a bit of pumice into the bottom of the applicator until it becomes clear and absorbed into the muneca. Don't apply pumice compound directly to the guitar.
Rub the muneca applicator against the surface of the guitar in a clockwise pattern, one small area at a time. Then reverse to a counterclockwise pattern. Don't use a straight forward pressure with the grain or it will pull filler from the pores of the wood. Add only a few drops of alcohol and pinches of pumice to the top of the muneca to recharge the polisher. You will need to wrap the muneca with new T-shirt material periodically.
Make a new muneca cover and gather a supply of several dozen extra pieces of T-shirt material for replacement covers.
Mix 8 drops of shellac flakes with 6 drops of alcohol, and soak this mixture into the muneca. Apply a drop of olive of oil, wet your finger and rub into the bottom of the applicator. Mix so that the muneca applies an almost dry patch of shellac to a piece of dry paper.
Apply shellac to the surface of the guitar. You should see a cloud or light film left behind as you apply the shellac using circular strokes. The cloud will appear and disappear as you wipe round and round. Reload the muneca with shellac when the cloud no longer appears. Alternate with a back and forth stroke to work the surface into a clear sheen. You will have to load the muneca four times or so to finish a full coat. This is called bodying.
Work shellac into the edges and corners of the guitar by working the muneca at a 45-degree angle. Work shellac into the slots in the headstock of the guitar at least twice before you finish this first session. Allow the coat to “gas out” for an hour, and then apply alcohol to a muneca and apply straight overlapping strokes to the entire guitar to remove excess oil from the bodying session. This is called spiriting.
Clean the muneca before each session and reload it. Body the neck, headstock, top, back and sides in each session, applying two complete coats, spiriting each coat. Allow a day between sessions, until you achieve the depth of coverage you want. Body the guitar for at least four sessions.
Leveling and Glazing
Wet sand the the surface with 400-grit sandpaper with a bit of oil to begin leveling the surface finish. Don't use too much oil or the sandpaper won't sand.
Sand lightly repeatedly until the surface is rough leveled. If you have sections that burn through, you may rebody the section, blending the coat into the surrounding area, then rebody the entire guitar.
Body the guitar three to eight more times before the final leveling procedure.
Apply one more body session, which should be about eight sessions for the beginner. Now, check the entire guitar for any defects, scratches, roll overs or holes. Use superglue to fill pores or holes, and scratches will require further bodying. Apply five more bodying sessions to complete the final leveling. Sand the entire guitar with 800-ultra-fine grit wet sandpaper. Use the sponge sanding blocks with a drop of olive oil. Sand in circular patterns.
Glaze the guitar with a thin cut of shellac by adding a few more drops of alcohol to your previous mixture. Use a clean muneca applicator. Work from heel to tail down the guitar in straight overlapping lines using olive oil the same way you did before. You will need to press the muneca applicator hard against the finish. This will make to make it smoother and harder. Once you've applied four or more glazing coats, allow it to dry for four days, and then polish with very mild rubbing compound to a high gloss.
Darker stain coats bring out the wood grain in the guitar better.
Use gloves and wear eye protection when handling stripper, stains, shellac or solvents.
Keep your work area well ventilated.