I worked very, very, carefully with a heat gun, set on low heat, and a putty knife (both of which we could lend you.) Most of the glass we have is about 75 years old and I don't know if the older glass is thicker or stronger, or if the glaze was just more rotten. I just wore garden gloves and kept moving the heat gun up and down the rotten putty til it softened and worked it out with the putty knife. If I ever do it again I would wear a mask to cut the lead paint vapors. If you have good, firm glazing, I'd leave it.
Okay. I think leaving it is the best option.
This started because I was losing the - ahem- "glazing" on one of the front windows. It was coming away from the glass and I could peel it off. I think they just used latex caulking. So I removed what I could just using my fingers.
So now I'm on a caulking-glazing-painting mission. I'd been thinking about the weatherstripping problem for a while.
Maybe I'll even get the living room windows open this fall.