Yellowing occurs over a number of years as the nitrocelluloselacquer clear coat is affected by ultra-violet rays. When lacquer yellows, it gives the effect of a road-hardened guitar that has seen lots of action at smoky juke joints, having lived a colorful life. Other factors, including sweat, smoke and general wear and tear also age a guitar, but for true yellowing, it must be done naturally or by applying a tinted nitrocellulose lacquer. The yellowing, relic effect, is desired by many guitar owners, especially those owning vintage re-issued guitars.
Keep the guitar in a sunlit room. Constant exposure to direct, or indirect, sunlight will speed the guitar's yellowing process, but not overnight. On average, it takes at least 20 years of normal exposure for a guitar's finish to yellow, but exposing it to sunlight can speed the process to half, or less, with a gradual yellowing. Be sure to rotate the guitar's position to entirely expose it. Exposing the guitar under UVB-313 florescent lighting is also recommended.
Apply a tinted nitrocellulose lacquer to the guitar. Remove all guitar hardware using a small Phillips screwdriver, keeping track of what goes where. If electric, remove the pickups and electronics. Remove the neck if it is bolt on, if not, cover it to avoid over spray using masking tape and newspaper.
Sand the guitar lightly, wet sanding it with a1000-grit sandpaper, progressing to 2000-grit. This scuffs up the clear coat, enabling it to hold more lacquer. Be careful to sand lightl so as not to affect the color coat underneath. Be especially light on the white edge bindings, if present.
Wipe down the guitar with paint thinner two to three times, sufficiently cleaning the surface. Place folded cardboard tubes underneath the bottom of the guitar to raise it from the work surface, making sure the guitar is steady.
Apply an amber-tinted nitrocellulose lacquer. Spray even coats in a consistent direction across the guitar's top and sides, holding the can 12 to 18 inches from the guitar. Allow to dry for at least an hour in between coats. Two to three coats of lacquer are recommended.
Turn the guitar over when dry, placing the folded cardboard inside the pickup and electronics cavities to raise it off the work surface. Repeat the process in Step 5 on the bottom and sides of the guitar, applying two to three coats. Allow the finish to set and dry for two to three weeks.
Sand the guitar, beginning with 1000-grit, working up to 2000-grit. This is a wet sanding process that brings out the shine of the lacquer. The shine will gradually appear, so sand until the desired gloss is obtained, being careful not to over sand to the point of scratching the guitar's color coat. When finished, wipe the guitar down and reassemble.
Natural yellowing only occurs on nitrocellulose lacquer and not on less expensive, water-based, lacquers. The amount of time for drying the lacquer before the final sanding depends on the temperature and humidity of the drying location.
Indoor direct sunlight does not normally get hot enough to affect the guitar. However, monitoring to see if it is overly warm is recommended. Indirect sunlight will not affect the guitar.