I have several wooden storm doors and storm windows which need to be repainted and have the glazing putty repaired. What is the best way to remove the old putty?

I've been digging it out with a utility knife and putty knife, but its very slow going and I ended up cracking the glass on one window when I pressed the putty knife down too hard. Most of the putty is extremely hard and brittle - some has shrunk and cracked away from the frame and is easy to get out, but many bits are still well bonded to the wood and glass.