There’re in a 30-year old upright piano. I can see the areas they’ve attacked. It’s in a house in Vermont that’s unheated during the winter. The exterior is black and varnished, but the interior–keys, soundboard, etc.–is pine and spruce and in its natural state. There’s been some fine sawdust on the floor coming from one section they’ve gotten in to on the underside of the bottom (not painted), and the bottom (which comes out) has more from where they’ve been at the soundboard. That’s also the case when you turn the piano around and look at the back. That’s entirely frame and soundboard, unpainted. Also when you lift out the keyboard–which you need to do from time to time for maintenance–you see sawdust from the underside of the keys.
If you read our POWDERPOST BEETLE ARTICLE, you’ll learn the best product for the job will be BORACARE. It sounds like you have plenty of unfinished wood that’s readily accessible so it should be fairly easy to do. Simply “paint” the application on so you don’t make a mess. And remember “less is more”. What I mean by this is that by applying small amounts over the same area several times, you’ll get more into the wood.
When treating an open, raw piece of lumber like what you might have on a deck, one can spray aggressively without care or concern. But because there is no doubt intricate workings inside the upright piano, you cannot use such a technique so painting it on would be the way to go. In the end you’ll get the same results; it will just take a little more time to do the job.
Lastly, make sure you understand the sawdust you’re finding is the “exit” holes where they’re leaving the upright piano and not signs of new ones attacking. This is a common misperception and it’s important to understand the difference. This is covered in our article.
Here are direct links to the information and products listed above:
Powderpost Beetle Article: http://www.powderpostbeetles.com/powderpost-beetle-control