Nitrocellulose lacquer is an evaporative finish, in which the nitrocellulose solids are dissolved in a solvent. When the finish is applied, solvent evaporates, leaving the solids as a protective surface that can be buffed to a high-gloss sheen. It has been the finishing product of choice for manufacturers of stringed instruments since the 1920s, and it is relatively easy to work with, although it does require considerable preparation and patience in its application.
Remove the strings from your guitar and discard them. If the guitar is acoustic, set aside the pegs holding the strings into the bridge. On electric guitars, remove the scratch plate and all the hardware using a correctly fitting screwdriver. Set everything aside in a safe place. Ensure that your workspace is clean and dry and has adequate ventilation.
Sand the body with 120-grit sandpaper, then work progressively through 220 and finally 320 to ensure a flat, smooth base on which to build up the new finish. Wipe the body with a lint-free cloth dampened with naphtha to remove all traces of wood dust. Mask off all areas of the guitar that are not to be refinished using the decorator's tape, including the sound hole on acoustic guitars.
Apply an initial coating of diluted vinyl sealer -- two parts sealer to one part lacquer thinner. This will help the subsequent lacquer coats stick to the body, creating a better final result. Spray the entire body of the guitar as evenly as possible with two coats. Leave the first coat to dry for an hour before applying the second coat. You can lightly sand the second coat after an hour with 320-grit paper to remove any roughness, but be careful not to remove the finish.
Fill the open pores of the wood with a pore filler product that matches the colouration of the wood. Water-based pore fillers are easiest to use because they dry quicker than oil-based products. Dilute the pore filler -- four parts filler to one part water -- into a thick paste. Work the paste into the grain with a flexible palette knife, focusing on small areas. Remove any excess filler immediately as you work over each area several times for a good finish. After leaving to dry for an hour, you can lightly sand any excess with 220-grit sandpaper before wiping it clean and applying another wet coat of vinyl sealer as before. Repeat this entire step, finishing with two sealant coats.
Spray the first two thin coats of lacquer, diluted 50/50 with lacquer thinner. Keep the spray moving to avoid drips and runs and allow at least 30 minutes between coats. Check the finish for any scratches or imperfections that can be sanded out with 320-grit wet or dry paper. Apply a third coating of lacquer and allow it to dry for at least two hours.
Spray a thicker "building" coating with three parts lacquer to one of thinner, and with 5 per cent lacquer retarder added. Spray two wet coats of lacquer, allowing five minutes between coats. For the second coat, pass the spray at right angles to the direction passed when applying the first coat. After an hour, examine the finish and sand any imperfections with 320-grit wet or dry paper. Spray a further two coats then leave to dry for at least two days.
Sand the dried lacquer layer flat with dampened 220-grit wet or dry sandpaper, wrapped around a sanding block. Working over the entire finish, gradually remove up to a third of the lacquer you've applied. Work methodically in small areas, and replace the paper frequently as it will get clogged easily. Any small pores should now be almost completely eliminated. Wipe the guitar clean and spray four further coats of building lacquer as in the previous step. Leave it to set for two days before repeating this whole step but using 320-grit wet or dry in place of 220.
Prepare a final coat mix of two parts lacquer to one part thinner, with 10 per cent retarder added. Spray two coats of finish over the body, spraying the second coat at a right angle to the first. Leave it to dry for at least an hour, then sand any imperfections with 400-grit wet or dry paper. Wipe off any dust before adding a two more coats. Set aside the guitar to dry and harden for up to 14 days.
Sand the dried lacquer using progressively finer wet or dry paper on a rubber sanding block. Begin with 600 grit, lubricated with soapy water and sand the body in every direction. Change to 1,200 grit and sand once more but work along the grain of the wood only this time. Finally, sanding with 2,000-grit paper will begin to bring some shine to the finish.
Polish the guitar to a deep, glassy finish with a polishing compound. Fashion a polishing pad with an old cotton T-shirt and buff the entire surface to bring out the shine. Add a little compound to the pad at a time and work your way over the whole surface of the guitar. Use an ultra-fine polishing compound to add even more gloss before rebuilding your guitar.
You may find it useful to practice your spraying technique on an old piece of wood before starting on your guitar.
Lacquers are available as aerosol sprays if you don't have your own spraying equipment; aerosol sprays do not need any thinning or retarder added.
The solvent in lacquer is highly flammable. Never use this product near naked flames, or smoke while working.
Always use safety goggles, gloves and a face-mask when sanding and spraying.