Once all of the inlay channels have routed and checked, it is time to glue the inlays into place. For this procedure, I prefer to mix up a batch of slow set 2-part epoxy to give me extra time to work and reposition the inlays slightly. I set the inlays so they are very slightly above the surrounding fretboard material. When you are satisfied with the positioning of each inlay, set the guitar aside and allow the epoxy to dry overnight.
Prepare the Mortise Filler:
If you are inlaying an Ebony fretboard, use Ebony dust and glue mixed together in a thickened paste-like consistency. There are also tints that are available that you can use, but honestly I have had much more success using actual dust. If you are inlaying in Rosewood, you are faced with a more unique set of circumstances. If you use rosewood dust with glue, you will find that the mixtures usually dries much darker than the surrounding fretboard wood. There are a couple of ways to address this problem.
First: Keep your inlay channels as close to the contour of the inlay piece as possible. This means tighter routing tolerances and more time to fit the inlays and a higher incidence of inlay being cracked unless you are extremely careful.
Second: Try to find the a light Rosewood board to gather your dust. Search for one that has much less dark veining and the resulting dust, once mixed with the glue, will dry a bit darker. Note that there are tints that are available for Rosewood fretboards too, but my experience, unlike with the Epoxy tints, has not worked out that well.
Making the Wood Dust:
Making the dust for the fill material is a job in and of itself. If you sand the wood with typical sandpaper, you will tend to get granules of sandpaper abrasive in the dust. Also the dust tends to be somewhat coarse. My preference has been to use dust that you generate with a fine file. This takes some time to generate , so be patient. You only need to make enough dust to tint the glue and don’t put so much dust in that the glue is too thick. You will have trouble filling properly and risk air bubble and gaps forming in the mortises.
Mix up the Epoxy and make sure to mix exact quantities of the resin and the catalyst. Also when you sir the 2 parts, go easy to minimize bubble generation in the mix. Now add the dust, mix the glue and check the color. Only add enough dust to tint the mixture to properly match the fretboard color.
Installing The Glue In The Mortises:
If you decide to use 5-minute Epoxy, you will only have the time to fill mortises of a couple of inlays with each mix. You will consume quite a bit of glue and dust mixture if you decide to go this route and overall, you will spend more time during the filling operation, even by using the quick set glue.
As for the Slow Setting Epoxy, I prefer to get down and dirty into the mess, and apply the mixture to the inlay mortises with my index finger. This is the best tool you can use as it is soft and will not harm the delicate inlays, and you can work the mixture down into the mortise much easier.
GLS Tip: If you prefer to minimize your premium dust mixture that you use, because it takes quite long to make the very fine wood dust, I use dust that I generate from an orbital sander with fine silicone carbide sandpaper about (400 grit) applied. This is easy to generate and since most of it (if you do this right), will be used below the surface, never to be seen. Then you can use your more valuable, very fine dust for the final 25% fill and use much less of that dust.
To fill the channels, take a bit of the mixture and apply it to the inlay area, now gently work the mixture into the channel by rubbing consistently with the ball of your index finger. For the first fill operation, work the glue down into the channel a bit so there is a slight depression of about 25% of the depth of the channel. This is very easy to do, by just pressing your finger around the perimeter of the inlay. I then clean the area of excess glue and set it aside to dry overnight.
Final Channel Filling Operation:
This is the final filling operation and you should mix up your premium dust with the slow-set Epoxy mixture. Mix as before, and fill with your finger once again. This time though, first work the mixture into the channels as best you can, to minimize air bubbles and then re-apply additional mixture so it is proud of the surface. Do not be too concerned if you cover the inlay as this will cleanup very quickly. Continue with all the inlays and set aside to dry overnight.
GLS Tip #2: If you are working with a fretboard that you have pre-slotted for frets, it will save you some time to quickly clean the slots of Epoxy glue mixture. While you don’t have to clean them perfectly at least give good channel so the fret saw can easily cut through the glue residue after the glue is dry. I use a section of old saw blade for this, but you can use most any material.
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