Touchup is one of the real dark arts of instrument repair, and one that exists in an almost entirely different universe from structural and functional repairs. A good touchup person needs an understanding of both the mechanical and chemical properties of the finish, as well as foresight into how both the finish and the repair will age. Vintage finishes require very different techniques than modern ones, and a bad touchup can really spoil the effect of an otherwise competent repair job, as well as the resale/market value of the guitar. There are some really brilliant touchup artists in the field, and their work can sometimes appear to border on magic.
Q: The top of my 1972 Martin D-18 recently got a good ding, about the size of a small pea; the finish is cracked and the spruce is depressed. Is there any way this can be repaired? —Richard Hannah, via email
A: Thanks for the question. Surface damage mostly falls into a few general categories—dings/dents, scratches, and cracks. Sometimes, a particular piece of damage will be a combination of these.
Dents are actual 3D impressions in the surface, usually caused by impact. These are most common in the soft wood of guitar tops, but can be easily found all over a well-loved guitar. In addition to crushing the cellular structure of the wood itself, a dent will either distort or fracture the film of finish that lies above it. Older finishes, such as shellac and nitrocellulose lacquer, are more likely to shatter, whereas newer polyester or polyurethane finishes are sometimes strong and flexible enough to follow the dent without breaking.
Scratches are topical abrasions of the finish film that don’t penetrate to the wood below. Often caused by picks, belt buckles, shirt buttons, and other everyday menaces, these can quickly make a brand-new guitar look worn—just ask any luthier who has ever exhibited at a guitar show! Fortunately, since scratches are topical, they are often easier to repair. Fine ones can often be made to disappear with a simple buffing. I use and recommend the Novus #2 liquid polish for anyone that wishes to try this themselves. Use a soft cloth and fairly firm pressure, and buff until the surface regains a glossy sheen. If you’re lucky, a small scratch will just vanish.
Deeper scratches, such as those sometimes left by string ends on headpieces or around the bridge, may need wet sanding with fine-grit sandpaper prior to buffing, to cut the finish down to the level of the bottom of the scratch and establish a new surface. This is fussy work, and best left to professionals. The film thickness of many fine instrument finishes is often less than .010-inch thick, and it can be all too easy to sand through entirely—an outcome that is very best avoided.
The final broad category is finish cracks, which are fractures in the film itself. (Please note that I am not discussing cracks in the wood itself in this case, just the finish.) This type of damage often occurs on guitars with thicker finishes, when the film becomes thick enough to be rigid. Wooden instruments move and fluctuate with humidity and temperature changes, and a successful finish is thin and flexible enough to accommodate that movement. When it is too thick, it will be unable to move with the wood, and it will simply crack instead. In the best cases, it will develop the fine-line checking pattern that is often sought-after in vintage guitars. However, in other cases, especially those that have resulted from sudden extreme shifts in temperature, the cracks are much less appealing to look at.
Finish cracks can often be dealt with by using capillary action to draw a repairing substance into the crack. On modern finishes, I have seen cracks and other problems vanish before my eyes as I applied a drop of water-thin cyanoacrylate (aka super glue). The glue is drawn into the crack, and bonds to both sides of the fracture, restoring strength and optical clarity to the film. The same technique works with vintage lacquer, though much more care is needed when using these glues around lacquer—their solvents will dissolve the surrounding finish, so any stray drop can cause a problem. Lacquer retarder, an additive designed to slow the curing of lacquer during spraying, is also sometimes used to repair topical fractures in the same way.
Getting back to your guitar: This is a compound problem, as you have both a surface disturbance (dent) and some associated cracking in the finish. When faced with this kind of touchup, I usually start by evaluating the overall condition and value of the instrument. If it is otherwise pristine and has the potential to be a collectible-grade guitar, then a fairly advanced approach to touchup may be required. However, if the instrument is a “player” with some normal wear for its age, then I may use a slightly less fussy approach. I would likely start by using butyl cellosolve or lacquer retarder to minimize the appearance of the checking. If the dent is shallow and the instrument is not museum-grade, I might then build up the surface back to level using medium-viscosity superglue, applied in fairly thin layers and allowed to dry without chemical acceleration. Once built up to above the original surface, this can be sanded flat and buffed to a gloss that will match the surrounding lacquer fairly well. Despite the fact that the wood is still dented, this kind of repair can often be surprisingly difficult to spot.
If the dent is deeper, a common technique is to try to use heat and moisture to reswell the wood back into shape before touchup. This can be very effective, but it must be done carefully—heat and moisture are also dangerous to lacquer finishes, and the wrong approach can cause hazes or blushing in the lacquer, dark spots, burns, and other problems. However, with experience and a light touch, a skillful touchup artist can often use a well-placed drop of water and some heat to steam the wood back into shape, dramatically reducing the depth and severity of a dent. I have even had moderate luck applying water and heat below the ding from inside the guitar, rather than coming at it from above.
Touchup techniques go far beyond what is described above, and the best touchup artists use all variety of tricks, from painting in grain lines to chemically aging either the wood or the repair so it will blend with a vintage guitar’s surface. This work lies at the intersection of chemistry, fine art, and highest-level woodworking, and often involves tricks and improvisations that reveal a brilliant mind at work.
Some repair techs, such as the truly amazing Iris Carr, bring the work to a level that defies belief, but not every instrument requires such a level of work. An experienced luthier or tech has usually seen dozens or hundreds of guitars with typical dings and dents, and has a vocabulary of techniques to minimize their visual impact. The nitrocellulose on your Martin is probably the most serviceable finish ever used on a production guitar, as well as the most common, so a competent repairperson can most likely make it much less obvious. Good luck!
This article originally appeared in the January/February 2022 issue of Acoustic Guitar magazine.