Well, last week I took some days off work. In a few days time, I had 2 windows repainted and working like new (for the most part). As Randy said, this is not an outpatient procedure. This is invasive surgery and can take some time. He thought with practice that I could repair 2-3 windows per day (without painting). I found that with waiting for paint to dry and other little things that pop up, I can do 1 every other day.
I'll try to breeze through the process of opening the window here (pun intended):
Put down a tarp around the area of the window. Paint chips will fly several feet and if stepped on can be ground into wood floors. It would also be prudent to use lead-safe work practices if you're not sure of the age of the paint layers that are present. Sealing the work area with plastic inside and out can keep lead paint chips and dust contained. A respirator should be worn during the paint breaking, window opening, and window cleaning. Wash your clothes separately than the rest of the laundry. Wet wipe surfaces that are dusty. Lead test kits are available. Usually 2 tests cost about $10 at the chain stores.
Use a utility knife (in both hands) to break the paint seal between the window trim and interior stops.
Unscrew the stops and lift the sash to expose any weatherstripping in the channel. I have galvanized interlocking and flanged weather stripping. You can see lots of examples of these at Accurate Metal Weatherstrip Co.
Hammer the long end of a thin pry bar scraper between the wood and the weatherstripping to loosen any nails and then pull them out with some dikes. You may have to get a little rough with the hammering. There is usually a nail at the bottom, center, and top of the weather stripping. Once one side is loose, you can pull the sash out of the frame.
Remove the ropes or chains from the sides of the sash by pulling any nails or screws holding them to the groove in the stiles.
With the window out you can remove the other side of weatherstripping much more easily. The weight pocket doors should be exposed once the weatherstripping is removed. You will need to open this door to re-rope the sash later.
The next step is to remove the parting stop/check bead/check stop/whatever from the frame. It is usually press-fit into a 1/4" deep or so channel. To remove it, pull the nails holding the meeting rail block onto the parting stop (this blocks air drafts where everything is meeting together on the sides of the meeting rails). Then use your utility knife (with both hands) to break the paint seals. Thirdly, hold the pry bar parallel to the sash and hammer it into the gap until it stops. Then gently pry the parting stop out. If you hear wood cracking, stop! Do this at the bottom and then the top gently working towards the middle. The last step requires both hands to gently pull and twist the parting stop out and around the meeting rail of the upper sash. You may have to reshape the weatherstripping ends on the meeting rail to work it out. They can be bent back into normal shape later with the pry bar scraper tool.
An alternative method to remove the parting stop that I have not tried is to use locking pliers to grip the check stop and pull it straight out. I'd be worried about marking/denting the wood. I think the idea is to prevent having to pry against the edge of a piece of wood and flattening a corner which is irreparable.
If you break the parting stop, most lumber centers carry pre-cut wood to replace it. I bought some and it was slightly narrower than the original parting stop. I primed and painted it with 2 coats. It could probably stand another coat or two to get it to a better thickness. Remember to cut a slant in the bottom with a miter saw to match the slope of your window sill.
If you need to replace your ropes/chains, the weight pocket door can be removed. It's usually got a couple flat-head screws at the top and bottom. Both weights are accessible from the one door. If you plan to reuse your ropes, it can stay closed. Most of my ropes were pre-broken or in bad shape. I am replacing them with Samson sash rope that I ordered from SRS Hardware.
Now it is time to loosen the (most likely paint-sealed) upper sash. Use the utility knife to break paint seals inside and out. You can gently tap on the meeting rail and gently wiggle the sash back and forth to break it loose. When it first breaks loose, expect a cloud of coal dust to make an appearance! Once it is loose, pull it all the way down past the weather stripping and off. The ropes will probably be in decent shape since the sash has most likely been painted shut for 50 years and don't get used that much even when not painted shut. It may be worth keeping them--in that case remove the nails and slide the knot out. It should be a big enough knot that it won't go through the pulley opening.
The fix it part:
Use a carbide scraper and sandpaper to smooth any bumpy paint. Prep, prime, and paint the window frame and sash. I taped the areas that contacted window stops so they would have less friction when installed.
Scrape the old glaze out with the utility knife. Anything that is more stuck can be removed with a heat gun and a razor blade. Keep your heat gun below 1100 degrees to avoid turning any lead into vapor. It does not take much heat to loosen glazing. A little heat is always better than too much heat to start with. Also, take care to avoid heating the glass. It will break if it is heated. A reducer and heat shield on the end of the heat gun will be helpful. Use one end of the razor blade while holding the other between your fingers to help prevent scrape marks on the glass.
Prime the glazing rabbets using a cheap paint brush and boiled linseed oil. Wipe off any excess. I used Sarco Dual Glaze so that I could rehang the windows and paint the glaze later with a ladder. If you use Type M, you will have to leave the sash in the workshop a week or so before painting it. That means you will need to cover the window opening with plastic, plywood, or foam board.
To reassemble, work in reverse of the disassembly steps. I have found it is easier to install the parting stop with the upper sash in the lowered position. I bought some beeswax to rub onto the stops and weatherstripping to help reduce sliding friction. Both upper sashes still require a small amount of force to move but my lower sashes slide up and down very easily.
Now my windows are fully functional and will last many more years. Don't forget to paint your glaze after the skin hardens and to overlap the paint onto the glass about 1/16"!