With the insects poisoned, the next step was to dig into the rot, and see just how extensive it was. At first, I wasn't optimistic that the window could be salvaged.
I attacked the rot with old hand chisels, just using hand pressure to slice out the worst of it. The sill itself was trashed, but amazingly the 2x4 running horizontally underneath the window was unscathed.
While I was chiseling out the rot, some insects kept flying up looking for home .. you can see one of them in the photo above. With Google's help, I ID'd this one as a carpenter ant. The original problem appeared to be the rot, and then the insects had chosen the rotten wood for home. But the insecticide that I used will take care of any that might try to move back in.
In the pic above, I have most of the rotted wood removed, and there's not much windowsill left. It was a two-piece design, with a glue-joint between the nose and the main sill. You can see some of that joint exposed at the far right of the sill.
At this point, the only way I could see to effect a good repair would be to remove the entire window frame, and rout the sill back to solid wood from the underside. But the installers assured me that the frame would fall apart if we tried to remove it. So then I asked about the cost of replacing the entire unit. The off-the-cuff ballpark estimate was between $2K and $4k. Gulp! On top of that, I had spent a chunk of the preceding week staining and painting replacement sashes for this very window. So I agreed that repair was the way to go, if possible.
After studying it some more, I realized that I could attach a replacement sill piece not just to the old sill, but also to that solid 2x4 underneath. Which would give more than enough strength, and also meant that a tight glue joint to the old sill wasn't necessary. I just had to get some semblance of a butt joint, and then I could attach it with epoxy and fill any gaps.
An oscillating multi-tool was just the ticket to trim the sill back to solid wood. I cut it back flush with the sash stops, using them for a blade guide. The remaining voids could be permanently repaired with epoxy filler. Good enough.
Actual Shopsmith work and photos to come in the next installment!