I believe what you and I call plumber's putty must be two different things.
The stuff my Father showed me (He was a plumber's apprentice decades ago) was a two part epoxy you kneaded into a putty and when it cured it was like rock.
I know however that 3M makes good poster tack, I've bought it at Michaels before, even seen it at Home Depot and Lowes.
<G> I hear you. I've got a tub of plumbers putty in the garage that's over 7 years old. Still soft and ready to use. I think what your dad showed you had a different name? Plumbers epoxy maybe???
Plumbers putty is the stuff you use when bedding a drain in the kitchen sink, lavatory, tub, etc... also comes in rope form. It need to stay soft and pliable to prevent leaks, which would happen if it dried out and cracked.
I googled hobby lobby and they carry an equivalent poster tack stuff. I'll get some next time I'm in that area.
Thanks everyone!