Cherry Guitar: Finishing

Work from 12/5/08 through 12/13/08

20081207_2008_12_guitar56.jpgTime for the stuff that really gives a project life and can cause hours of aggravation: the finish.  There are two major options for finishing a wood guitar: a clear film finish (traditionally varnish or lacquer) or oil.  Oil finishes are super easy to apply.  You just wipe on 10-12 coats of oil, letting it dry overnight between coats.  The surface of the wood has to be perfect though because the oil will enhance every missed sanding scratch.  Oil doesn’t give the guitar much protection either.  A film finish is much tougher.  It was mainly for this reason I chose lacquer.  I chose a water-borne lacquer called KTM-9 that came highly regarded by other home luthiers.  Before I could spray on the lacquer I needed to do some prep work…

20081205_2008_12_guitar33.jpgFirst I had to install and dress all the frets.  It sounded all a bit nerve-wracking at first but ended up being a fairly straightforward operation. Fret wire (medium height – medium width) comes in 18″ long sections.  The barbed tang gets hammered into the slots in the fingerboard and should be enough for a permanent installation.  My books both recommended putting a few drops of wood glue in each slot.. just in case.  The frets were trimmed flush with a nippers and filed even with the fingerboard.  The frets are never perfectly level so they get sanded even and then shaped back with a fine triangular file.  Then they and the ebony fingerboard get polished using sandpaper, starting with P400 grit and working up to P1200.

20081206_2008_12_guitar43.jpg20081206_2008_12_guitar45.jpgThe trench for the nut was cut and then everything, except for the polished fingerboard, was sanded to P240.  To prevent splotching, the softer top was further sanded to P320.  The top of the fingerboard, the location of the bridge and the soundhole were masked off with tape.  The entire guitar was then sealed with a very thin coat of marine epoxy… Yup, the material recommended by the maker of the KTM-9, Grafted Coatings, to fill the pores and seal the wood was my old friend two-part clear epoxy.  Yeah, I think I had a gallon or two of MAS on hand… I mixed up about an ounce, thickened it with some aerosil (fumed silica) and squeegeed on a very thin coating.  After it had cured overnight it was leveled with P400 dry sandpaper.

20081207_2008_12_guitar52.jpg20081208_2008_12_guitar57.jpgTraditionally a nitro-cellulose lacquer is used but the reducers are high in VOC’s and are quite nasty!  I went with a water-borne lacquer mainly because I could safely spray it in the shop without killing any one.  I set up an area to spray, hung the guitar on a wire hanger and sprayed on the first 6 coats of lacquer with my HVLP gun.  One coat was sprayed every hour. It was left to dry overnight and the next day was leveled with P400 dry paper and the last 4 coats were sprayed on.

This KTM-9 was interesting stuff.  It was thin like water.  It was very easy to spray on too much and have sags and drips.  You couldn’t spray on extremely light coats because they would dry too fast and not level out…  I got used to it after the 4th coat after increasing the air pressure slightly and dialing back the width of fan.  The gun really needed a smaller nozzle for better atomization… oh well. I knew it had to be wet-sanded and polished anyway which was the last operation.

20081213_2008_12_guitar40.jpg20081213_2008_12_guitar41.jpgLike most lacquers, this stuff shrinks like crazy when it cures.  They recommend letting it cure for a minimum of five days, two weeks if possible.  Christmas was coming fast so I brought up stairs into our office where it hung in a nice, warm room.  Last saturday, the 13th after hanging for five days, I brought it back in the shop and wet-sanded the lacquer level.  I started with P600 and worked up through P1200.  I then power-sanded it with a Festool S2000 pad on my RO125 sander which brought it up to a semi-gloss.  For the final high gloss I used felt pads loaded with Mequire’s automotive rubbing compounds.  I used #84 Power Compound, then #9 Swirl Remover and finished up with #3 Glaze and some silicone-free wax.   It looked like a mirror…

The final bit of finish work was to remove the masking tape and to oil the ebony fingerboard.  All that’s left now on this project is the final setup…