replace rotting wood

You are in trouble---Generally if the sash is rotting the door is done--junk.

Post a picture if you can---I repair a lot of rotting window sills but the cost in time and skills and tooling to rebuild a sash has always made replacement the better option.

What he said, but most manufacturers guarantee such things for 10 yrs. If older than that, replcement is usually the option. There are wood epoxy systems that might prolong the inevitable, but you need to dig out the rot, fill and sand the epoxy.