make plumber's putty soft
I’ve had mine for about 2 1/2 years and still seems playable. The manufacturer of my “wood filler” makes a “Thinner” which they claim “Restores solvent based wood fillers to their original consistency.”

BTW, where I live there’s an ‘ol finish carpenter that we all admire how close his filler in the nail holes match the stain of the wood. A few years ago I stuck around on a job and watched him work. After he cut the trim boards, he saved the sawdust and mixed it in the wood putty. Clever.  Now, I think, there is putty that can be stained to match wood.