During the dryer winter months with forced heating in temperate regions like here in Norway and back in my homeland Canada, I see the predictable increase of cracks in acoustic guitars that have not been maintained at the proper 40-60% humidity range, with late-February, early- March being the peak. Cracks to the spruce or cedar tops of good, solid-topped guitars are the most common, followed by cracks to the backs or sides of guitars made with mahogany or rosewood (the most crack-prone), while maple backed guitars don’t seem to suffer the effects of shrinking (causing the cracks) due to severe dehydration nearly as much. These are usually fixed using the traditional glue and cleating methods.
But what about fingerboards?…The first indicators of dehydration in the fingerboard are the appearance of slightly protruding, sharp fret ends as the wood shrinks but the metal frets, don’t. In some cases re-hydration can restore some of the wood back, but only so far, and in all instances corrective filing of the uncomfortably sharp ends is the only answer. But the fingerboard wood itself is sometimes another story. Cracks, almost always on the ‘tongue’ or upper end of the board, does happen in severe cases and like all cracks, travel with the grain and will spread further – not if, but when – unless we stop those in their tracks at first sight.
Of the three main types of common woods used in guitar construction for fingerboards (rosewood, ebony and maple), ebony is without doubt the most crack-prone due to it’s sheer hardness and density. You do see them happen to rosewood sometimes too, and in my entire repair career I’ve only seen one cracked maple board.
Our demonstrative subject is a new Larrivee D-03R, that came to the shop quite dehydrated with a separating rosewood back and this: the classic ebony tongue crack. This is my general method for making this go away…
Here’s our initial diagnosis: a 94mm long tongue crack, and oxidized frets to add insult to injury.
First, frets 15 to 20 must be removed to gain clear access. Heating the fret before pulling them helps a hugely in avoiding chipping from the tangs biting into the ebony, which is very prone to this.
Carefully ‘walking’ the nipper from one end to the other in the traditional pulling method.
The area is now de-fretted cleanly as shown, cleaned with naphtha and ready to begin the crack-fill process.
We need to get ebony dust, and the best way to do this is by keeping scrap pieces of the appropriate matching wood around. In this case, I get some fine dust from this old ebony violin fingerboard using a flat mill file.
Before we get too far, we need to use teflon dams to prevent fill and glue going into the fret slots as much as possible. This little step will obviously save on clean-up time a bit later on, and eliminate the need to do any slot cutting near the crack area. Teflon is used as a dam because glues will not stick to it.
One section at a time, the crack is now filled with ebony dust and patted down until it’s completely filled. Then a small portion of dust is also laid along the crackline forming a mound, which will eliminate any voids caused by sinking and shrinkage after the glue is applied.
Now we are ready to start gluing…
Very low viscosity CA glue is carefully and accurately applied, letting it soak into the dust and sink down into the crack. Despite a fast setting time of CA glues on the surface, you should let this harden fully down into the crack for at least an hour before removing the dams and continuing.
First is scraping down the now-hardened mass to be as close to flush as possible to the surface of the fingerboard. For this I use the usual crack, chip and dent-fill scraper: a straight razor blade burred on one side using a screwdriver as a burnisher, the end corners nipped off, and then covered with making tape leaving only the exact amount you want exposed as a scraper for fine work. With this method, I can carefully scrape down to an almost flush .004″ before sanding!
After scraping, you’ll end up looking like this. Much better already
Now it’s time to sand it all down nice and flush. I’ll spare you most of it here, but first I start with a 20″ radius block and 320 3M gold sandpaper, followed by 600, 800 and then 1000 blocks. When this stage is done, it should be very smooth and ready to start re-fretting.
Each fret slot must be thoroughly cleaned out before trying to reseat each fret. My favorite tool for cleaning fret slots is an old dental pick, fantastic for the job. I was still pulling out bits of crap even after I thought I was finished with just a brush. You cannot overlook doing this step…
I will install the frets using a normal fret hammer, and then check for levelness, seating them down as I go.
When the frets are level to my satisfaction, I will glue them down to prevent any lifting in the future.
Invariably the fret ends are still sharp and poking up just a touch from the initial dehydration, so I will routinely dress the fret ends on the entire neck while I’m at it.
Finally, after thoroughly cleaning the oxidization from the frets and giving the bone-dry fingerboard a much-needed treatment of lemon oil, we arrive at our final result : an practically invisible repair and a happy customer!
Good as new! Now keep your guitars properly humidified folks.