Need advice for masking windows

[Tom M King]

But there is still a place for the single edged razor blade. That bedding caulking in the previous picture being separated from the glass, was first cut with a razor blade because it's thin. If the bedding caulking is only cut with the chisel, as in that picture, it can be pulled out from where you want it to stay. The razor separates the part you want to get rid of. The sharp chisel cleans the glass while parting the bedding caulking. The chisel will work, but being so much sharper than the razor blade, it's too easy to scalp wood from the sash.

I use cheap latex caulking for bedding. It is easy to get excess off like this, but still makes a decent enough gasket so that the paint on the inside of single glazed sash can bond to the glass without having a dip between the glass, and sash, to hold water. Interior surfaces of sash, or glass doors, with single pane glass need to be coated with an exterior coating. They will need to shed water at some point.

The sash in these pictures is from 1850.