If you are building a guitar or refinishing an old guitar, applying the varnish is the finishing touch. Varnish is a combination of oil and natural resins, such as rosin or amber, that provides a protective and durable guitar finish. Varnish is resistant to moisture, heat and scratches. Applying varnish is a time- and labour-intensive process. The best results are produced by applying 10 to 12 light coats of wiping varnish by hand, with either a brush or rag.
Set up a work area for the guitar in a well-ventilated and dust-free environment.
Sand the guitar with 220-grit sandpaper to prepare the surface for the varnishing process. Sand the guitar with #0000 steel wool to remove the sanding dust from the pores in the wood. Wipe the guitar with a tack cloth to remove sanding dust. Vacuum the surrounding area with a vacuum cleaner to remove the last remnants of dust.
Dilute the varnish with white spirit. Use a 50-50 mix of varnish and white spirit or turpentine. Begin with a small amount in a small container. You might have to make adjustments to the mix depending on the wood of the guitar.
Decide whether you want to apply the varnish with a brush or a rag. Use a clean lint-free cloth if you are applying the varnish by hand. If you decide to use a brush, use a tapered chisel-edge brush designed for varnish.
Hold the guitar in the middle of the neck. Apply a light coat of varnish to the back, front and sides of the guitar. Avoid varnishing the neck at this point. Use the neck to rotate the guitar as you varnish the body. Begin on the back side of the guitar. Apply the varnish to an area with one smooth movement. Don't go over an area once the varnish has been applied. The goal is to minimise the number of overlapping lines.
Hang the guitar on a hook. Apply the varnish to the neck. Allow the varnish to dry overnight.
Lightly buff the guitar with #0000 steel wool. Wipe the guitar with a tack cloth to remove the dust. Apply a second coat of varnish. Repeat the process eight to 10 times. Buff the finish with steel wool before each application.
If the varnish puddles or runs, it is too thin. If the varnish is too thick or sloppy, it needs to be thinned. If the varnish is too thick, it will dry too quickly and leave bubbles. You should be able to apply one coat before the varnish begins to dry. If the varnish begins to dry before finishing a coat, thin the varnish with white spirit.