Bedding is the foundation of a grand regulation and piano manufacturers put a lot of effort into providing a flat, stable surface on which to construct this foundation. But keybeds, as they age, go through whatever extremes of humidity come their way, whatever neglect happens over time, and whatever unlooked for additions get deposited. And we cannot bed to a glue drip or anything else that stands above the smooth surface of the keybed where the keyframe needs to touch down.
The human hand is a sensitive feeling machine, designed to detect surface texture and quality. If asked to "feel this", the first volunteer will be our hand or, perhaps, the fingers of our hand. Some prudence and care should always accompany committing these sensitivities to a surface. In the case of a wooden keybed, splinters can be a hazard, or a sharp edge of something embedded in the surface. On the other hand, if our need is to hunt down a bedding spoiler, here is our best tool.
Generally, to fit the front and back rails of a keyframe to their supporting keybed, remove material from the keyframe's areas of contact until all areas contact. But imagine that, with great industry, we have carefully removed material over a glue drip and succeeded in completing the fit. What happens when the soft pedal is engaged? Imagine we have bedded to a lip of keybed stile projecting slightly above the level of its rail. In either of these cases, the keyframe either won't shift or the bedding will be considerably undone.
I once worked on a Mason & Hamlin BB from the early 1900's with a keyframe that flunked all the initial tests. When I removed the action, a discouraging mess explained a lot. A vacuum cleaner and action brush removed dust and debris, a hand-held razor blade skimmed off spillage and overspray, a chisel knocked off glue drips (yes, it had all of these features), and 220 sandpaper cleaned up the surface, with a little help from the razor blade, this time used as a scraper. Much of this mess came from technician carelessness. The piano had been rebuilt, maybe more than once, and there were offcuts of piano wire in with the debris. And the keybed had scrape-trails from under-keyframe grit.
Anyway, when I was done, the keyframe fit perfectly! No further bedding was needed. It obviously had not been touched since completion in the factory. And how impressive, the understanding and craftsmanship that designed, made, and fit this piano and its action!
Then, suddenly, we are the next collaborators, bringing what we can to most benefit our customers. What can we do?
So, one small thing. I always hand-dust the keybed before action insertion. Even in a clean, young piano, a little something will often be found to justify the effort.
Next week: How Flat is the Keybed?