Humidity and Warping Fingerboards

[Tom OBrien]

I don't know about other places in the U.S., but the past 6 months of this year have been unbearably humid in Chicago, with all the attendant troubles this implies for building and finishing. It feels like I could write a book-length treatise on jigs, fixtures and work-arounds that I've had to implement to produce instruments that are playable and look marginally appealing. Here I just want to share one of the hurdles erected by high humidity – warping fingerboards.

Obviously, fingerboards warp for a variety of reasons, and one of the most common is due to the use of water-based glues when attaching the fingerboard to the neck. There is a lot of information out there concerning this issue, so I'll let those more experienced authors speak to that topic.

Recently, I've had most of my fingerboards warp for reasons that can't be blamed on Tightbond, or LMI White Glue. The culprit in this case is a shop environment with a relative humidity (RH) above 65%, combined shop procedures that were established during more normal conditions where the RH was below 60%. What I've learned is that my procedures had to change to accommodate the new humidity.

Change #1: Seal the fingerboard when it arrives in the shop. This is an easy fix and helps significantly. I use a very thin fingerboard oil that's formulated using linseed and special drying agents. I apply it on the surface both sides of the fingerboard. The oil serves as a prophylactic against moisture in the air. At first I was concerned about compromising the gluing surface, but this kind of oil finish normally soaks into the wood and away from the surface rather quickly. As an added precaution, you can give the gluing surface a quick wipe with naptha before attaching the fingerboard to the neck.

Addendum from Ed the Pianoman (who knows much more than I do about these things): Always seal the end grain of the fingerboards because these are the "lungs" of the wood. He also has a preference for using shellac to seal woods and maintains that my procedure would be improved by using shellac rather than the thin oil – wiping the gluing surface with denatured alcohol before attaching it to the neck.

Change #2: Build a clamping caul for gluing the fingerboard to the neck that does not allow any room for warping. This is a lot more time consuming, but once it's done, the caul can be used again and again. I use a board that is slightly longer than the fingerboard to make the caul. I build them out of laminated maple pin blocks that have been discarded from old pianos. These boards are hard like metal and very unlikely to warp due to the way they're manufactured. I cut slots across these boards using the table saw according to the scale length of the instrument I'm building. The frets will then fit snugly into these slots during the gluing process. Then I sand a negative radius into the caul to accommodate the positive radius of the fingerboard. In the end, I have a very fine clamping platform for a fretted fingerboard to be attached to the neck.

Pictures below.

[Dave Bagwill]

That's good stuff, Tim! Thanks a lot.