My house had a door removed by previous owners. The recesses in the jambs for hinges and catch were filled in with pieces of board (I don't know exactly what, but not solid timber), and painted over.
I've hung a door there, removing the filled parts and placing hinges in the original locations. After a few months the door has sagged, and I cannot tighten the screws in the top hinge - they just turn in place.
I was advised at my local home/hardware store that to fix this, I should spray some wood hardener into the screw holes, then when that has dried, fill with builder's bog and re-drill.
There's a similar problem described on this site, where the answer suggested gluing toothpicks into the hole, using longer screws, or drilling larger holes, gluing dowel in and re-drilling. These options sound easier.
So (finally) my question:
Do I only need to go down the wood hardener path if the frame is rotten? And if so, how can I tell if the frame is rotten? It doesn't seem to be rotten from looking at the visible surface - can it still be rotten inside? It's an internal door, and I have no other reason to suspect rotten wood.