Originally Posted by
Simon Barden
Boiled linseed oil polymerises, so you’ll get a hard finish in the fretboard surface if you use that. Some people use unboiled linseed oil, which can harden over time but is mainly a fairly thick oil. Lemon oil is the most common oil used on fretboards and mentioned in guitar care articles, but other similar mineral oils can be used. It just needs to be absorbed by the wood to make up for lost moisture, but not remain on the surface so that it makes your fingers oily. As a result, lemon oil or mineral oil is light enough to evaporate over time, which is why you need to oil the board fairly often. Here in the UK you can normally get by by doing it once a year, but i know in warmer, drier climates, you may need to do it every three months.
I don’t know enough to know if bowties will do much good in this instance. I think you’ve had the worst of the shrinkage so sealing the surface should be enough to keep things stable. But you do want something firmer than wood glue, say epoxy, in the crack to give a more rigid filling that can spread any compression loading between the pieces of ash.