You would have to immerse your fretboard in WD40 for a week before encountering any adverse effects.

The whole point about Fast Fret (whether augmented by WD40 or not)is that you wipe it on and WIPE IT OFF. The minute amount that comes in contact with the fretboard isn't going to do squat to your frets or your fretboard.

I have always used it on all my guitars , and the fretboards are still as hard as they were thirty years ago.

I only wish I could say the same for me ...