My latest project is my large guest room. We knew that one of the walls was 'false' and concealing a pair of windows (you could see the windows from outside).
Demolition - I rented a dumpster
and hucked all the debris out the window - very satisfying. Here's a pic of halfway through the demo.
Peeled off three layers of wallpaper. They conveniently PAINTED between the first and second layer making it quite a PITA to scrape.
They also PAINTED the hidden windows shut from the outside!
You can easily see the paint on the upper sash. It took a bit of patient work to remove the sashes. I kept the interlocking weather-stripping in place because it was in good shape and engaged with the sashes nicely.
I removed all the old paint, sanded and rehabbed the sashes. This involved re-attaching loose muntins and re-squaring a corner or two. The upper sash is still raked out of square by 1/4 inch, but it opens and lowers nicely in the window. The lower sash is perfectly square and also performs nicely.