The author believes that windows should be replaced if they're falling apart, but that most nominally sound windows old enough to have sash weights deserve an upgrade rather than a toss in the dumpster.
Since old windows of this type typically have some lead paint on them, I set up a lead-safe work space around each of the window frames, and take appropriate safety measures while preparing the sash and stops for weatherstripping.
A spring clamp prevents the sash cord from dropping into the pocket; on the window shown, the trim was in bad shape and was removed for replacement, allowing the author to insulate the cavity with XPS and spray foam.
To improve the window’s operation, old paint is removed from all running surfaces; here the author uses an infrared paint-stripping tool.
Paint is also removed from the inside of the window frame and the edge of the stop; raw wood surfaces will be waxed.
After years of weatherstripping old windows and doors, the author has settled on three seal profiles for most jobs: a polypropylene brush seal for the sides; a 1/4-inch tube seal (white or bronze) for the meeting rail, and a white 3/16-inch tube for the bottom.
The bottom edge of the sash must be trimmed by about 1/8 inch to account for the thickness of the weather seal.
Using a plunge router, the author centers the slot for the bottom tube seal 5/8 inch from the face of the sash.
The side brush seals are placed as close to the edge as practical — about 1/4 inch. Any closer and the thin strip of wood left behind may break off.
Before the weatherstripped sash can be reinstalled, the stool must be scribed and trimmed to allow for the thickness of the brush seals on the face of the side rails. A 1/16-inch-wide gap, scribed with a carpenter’s pencil, allows for expansion.