I have been working off and on a 86 year old home. This home feature lots of 12 over 12 windows. I have been sanding and caulking these for quite some time.
Howdy! For finished woodwork I have found that scraping does a much better job, and no sanding dust!
Where have you been caulking, the panes, or because of wood shrinkage?
Mostly scrapping as it is more effective. The bulk of the caulking is along sash stops where cracks and gaps have occurred. During the summer I was reworking a lot of the pane's putty outdoors. This home suffered some damage from a hail storm and a number of panes were replaced. Unfortunately the crew that worked on them were not very skilled with glazing compound. It looked like they applied it with their elbows.
So, is the caulking for air leaks, or for cosmetic look on the outside?
I Love my scrapers!
A little bit of both...seal and cosmetic.
Have you thought of using a piece of trim, on the outside, and a compressable insulator where top of window meets frame?
This house has external storm windows
When I see aluminum storms I just have to look and see if the weep holes were caulked over or not.
Good luck with your project. Older houses are such a blessing in some ways, but definitely not for the project challenged. It is always a work in progress.