I scraped off everything spongy, leaving a 1.5-inch-deep cavity under the inside sill which I filled with urethane foam. I painted the outside sill with a Zinsser fixative-slash-primer - I plain love everything Zinsser makes, really a better life through chemistry kind of company!
The result was something pretty sturdy for the next few years; the wood that remained was still pretty solid. Here you see that intermediate stage after about three coats of that primer. It's thin stuff that soaks into the substrate and makes a solid surface. I love it.
That's the view from the outside before any paint was applied (I don't think you've seen the north face of the carriage house yet, and that's the top of the outhouse, which is built exactly like a brick outhouse). Once this much was done, I filled in the cavities with caulk and painted the heck out of it; two coats of my oil-based exterior trim paint. The result looked like it hadn't been neglected since the Nixon Administration, so I figured it was time to pop in the window.
Then I went into hibernation.
Well, fast-forward to now. Unfortunately, I got a little out of the habit of photographing everything, and it appears I didn't take a picture of the untrimmed interior - suffice it to say it had yellow foam sticking out, and you can see what the interior trim looked like. My wife put down the imperial foot, and beautification thus became the order of the day. Fortunately, the paying work had come to an abrupt halt on April 16th, for the usual lack of any discernible reason (the translation industry acts seasonal - but scaled to a year from some other planet, perhaps one in a binary system without a stable orbit). The upshot: I had both motivation and time. It was time to trim.
Trimming was complicated by the fact that the window was actually an inch too narrow for its frame (I didn't want to spring the extra $100 for a custom size for one lousy inch). So instead of a simple quarter-round trim on the edges of the pocket window replacement, I used 1x1s left over from the roof job last summer. You can see here that the trim paint had to be stripped, too - like all the trim in the carriage house apartment, it's latex applied on top of oil. Sigh. Without primer, of course, so it just peels off. Here you see it peeled off.
After much heat-gunning, the oil coat underneath was also removed (well, mostly), and the fully-trimmed window is now ready for painting. An unfortunate lens artifact makes it look like the carriage house is a lot less square than it actually is; the window does not actually bulge like this in real life.
Here you can also see the blue plastic wall tiles of the bathroom. A much later followup job will be to take those off, scrape their glue, and paint the entire wall with a resilient bathroom-appropriate paint. They're pretty horrendous.
Zinsser 1-2-3 primer sticks to anything, including the residual oil paint left on this trim, and since I'm using an interior latex, priming is important. Priming also shows us just how many nail holes have been knocked into this window over the decades; so I caulked them all up and laid down a thin bead along all the seams of the trim as well.
The next day, the topcoat - et voilà! A lovely trimmed window just like Mama used to make. Now I only have one window (the kitchen window) to replace, and a total of four to trim and paint; I've started stripping the paint on the bedroom window this week.
This window is literally so nice it gives me a thrill to look at it, even a week later.