I’ve found I can’t handle radius sanding blocks, so I never use them. Even if I hold my flat block at a slight diagonal, it still cuts a perfectly straight path, which I can view easily by the dust that packs into the fret slots. When I’m being especially careful, I pencil a few “witness marks” to make it even more clear. If I have really bad areas to level, I’ll scrape them first to avoid unnecessary effort with the sanding block. As I sand, I “roll” the block from side to side to match the radius of the fingerboard as closely as possible.
I may use a circular sanding motion with coarse paper to grind down a hump in one area of the fingerboard, but I’ll always finish by sanding with the grain. I’ll sand until I feel the sandpaper begin to load. At that point it suddenly feels as if the fingerboard has been greased. We’re only talking about 25-30 seconds of sanding per piece of sandpaper, using each grit available in succession (80, 100, 120, 150, 180, 220.)
I vacuum out the fret slots after each piece of sandpaper so I can see my progress and avoid excess loading. My sandpaper of choice is no-load stearate coated silicon carbide A-weight. I’ve not found any difference in performance, so I buy the cheapest Korean stuff and throw it away with no regrets when it begins to load up. The coarsest I use is 80-grit; I finish with 180 on rosewood and 220 on ebony. I choose my starting grit based on the amount of sanding I think will be necessary -- anywhere from 80-150. My goal is to level the board and give a perfect substrate for the new frets without removing any more wood than necessary. My final sanding scratches are removed when I polish the frets.
I have my fingers spread as far as possible supporting the neck from underneath and I’m raising the peghead off the bench. When I sand, the neck “floats” as I support it. That way I’m not bending the neck as I sand, no matter how hard I press with the sanding block.
If I wanted to introduce a “reverse bow” I’d support neck by resting the peghead on the bench. Then, as I sand, the neck would flex forward, causing me to take a little extra off the fingerboard at the nut end. This would be helpful in correcting a seriously flexible neck that pulled forward under string tension.
I can create a little forward relief by supporting the neck in the center and using a heavy lead shot filled bag to flex the peghead backward as I sand. This is especially useful in refretting classical guitars. Definitely one of those times where judgment from practice and experience comes in to play.
If my method seems a bit uncontrolled, bear in mind that the stiffness of the neck will be somewhat unpredictable as a result of changing the compression in the slots after removing old frets and installing new frets. I count on my ability to judge the way things feel and look as I go, and I am always ready to redo the work completely if it doesn’t come out just right.
Preparation of the fingerboard is so important to me that I’ll put in whatever effort it takes to get the surface as perfect as I can. I like to leave it to the customer to decide whether to fill very deep fingernail damage between frets. Most of the time I recommend not filling those divots, because I think that they look ugly after they’re worn later, like little islands of synthetic material. A little honest wear gives an instrument character, don’t you know.
10. Create drop-off” at end of fingerboard. Next, I apply 4-5 layers of masking tape at 6th fret to raise & support the end of my sanding block. Then I sand the upper end of the fingerboard with half-length sandpaper in same grit sequence to create a “drop-off” over the body.
I like to see the end of the fingerboard drop about .015” over the body to compensate for the neck pulling upward under tension. I find that it provides the best overall contour if the drop-off starts 2 frets out from the body: at the 12th fret for a 14-fret guitar, etc. I think there is too much chance of that slight ”hump” at the body if the drop-off starts right at the 14th fret.
11. Check with straightedge for neck straightness and drop-off. I like to start with the neck perfectly straight through its flexible portion (nut to twelfth fret) so the string tension will pull the neck forward just a little to create the proper relief. If the body flexes a little under the string tension, the drop-off may be reduced or eliminated.
I don’t usually look under the straightedge, but instead listen for the straightedge bumping the fingerboard as I rock it back and forth pivoting on the apex I’ve created at the 12th fret.
12. Bevel the upper edges of the fret slots. With a small triangular file, I bevel the slots to make it safer for the next guy to pull the frets without chipping and to avoid chips when cleaning out the slots and driving frets in. Beveling the tops of the slots helps drive frets in more easily, but that’s not my reason for doing it. If I really want to drive extra-tight frets, I’ll file the tang of the fret to a taper at the bottom.
13. Saw or pick slots to clean & deepen. My favorite fret saw is actually a Japanese razor saw that cuts about .018” kerf. It’s insanely sharp and cleans & deepens slots with ease on the pull stroke. I cut inward from each end of the slot to avoid chipping out. I’m not too concerned with the saw kerf actually matching the size of the fret slot, as long as it’s a just little narrower. When I deepen the slot accommodate new fretwire, I’m usually cutting below the part of the fret tang where those little bumps grip the slot, and I don’t want to widen the slots.
I’m more cleaning than deepening the slot, so my new cut doesn’t have much influence on the fit of the new fretwire. If my choice of fretwire is limited, I’d rather file the tang thinner than widen the slots. (When in doubt, the conservative approach is always a good choice -- you can always go back with different frets, but you can’t replace the wood in the fret slots if you widen them.)
I’ve made a special little pick for cleaning out the slots of bound fingerboards:
It’s hollow ground from a violin knife and is a hook that cuts and scrapes like a chisel only at the point. I’ve rounded all edges except the very chisel point. I hold it like a pencil with my right hand, and grip the back of the handle with my left. Using my right palm as a fulcrum, I can bear down very hard, dragging the pick along, deepening & cleaning with no chance of slipping or widening the slot:
Unnecessarily widening fret slots will limit future refretting.
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