Our current house has all double glazed windows but I found myself restoring a very old door to use on the chicken coop that had a glass 12 light panel inset. Most of the glazing putty was gone but thankfully the glass panes were not broken. The old style glazing points, which are either diamond or triangle in shape and flat, sharp metal were mostly still holding the glass in but I did put in some new
glazier's points just to be sure. One of the most important things when reglazing a window, which can be done with the window sash still installed on the house, is to remove all of the accumulated dirt in the glazing channel so when primed, it sticks and seals. Here's the simple steps to reglazing a window: 1) remove all of the loose, old glazing putty, being careful not to chip the glass. Do the whole window once, take a break then come back to it and you'll find more loose stuff. Remove it all. Any that seems really secure can be left, usually the top edge of the glass since this rarely sees water damage. 2) clean out the dirt and crumbles of putty. This can be a challenge along the bottom edge that gets the most wear. I'll scrape it then go back with a small wire brush to loosen the dirt. Vacuum up the dirt dust and debris with a shop vac, making sure the whole channel is clean. Do not use water to clean unpainted wood. 3) paint primer in the channel where the putty will go. I usually prime the whole window at this point. Don't worry about getting primer on the glass, it'll scrape off later.
4) apply the glazing putty or caulk. Be patient! I use a tool (in the picture) to smooth the putty to a nice angle that will shed water. Use a wet finger to smooth where the tool overlaps the putty and creates a bump or drag mark. Leave any excess putty on the window as long as it's not touching the glazing putty that was just applied. It'll harden and be scraped off later. 5) after about 3 days, the glazing putty is hardened so it can be trimmed if needed, like if there's a bump or tool mark. Use a sharp razor blade for this. A sharp box cutter held at an angle also works but keep enough material to ensure shedding of water. Once satisfied with how it looks, it can then be painted with the top coat. I've had situations where I forgot to paint the window after the new glazing putty is in and it makes more work since you then have to clean what you applied of the dirt and dust collected on it before painting (it stays a little tacky so it collects dirt fast).
With a little practice and development of technique, the exterior of vintage windows can look new and be more efficient, or new life can be given to an old door headed for the scrap pile.