Door-frame rot usually starts at the sill and lower jambs where water sits. Permanent repair means replacing soft wood and fixing the water path, not only filling the cavity.
Confirm extent
Probe the jamb, sill, and brickmold. Soft sections often run farther than the paint blister suggests. Check the subfloor and threshold too.
Cut back to sound wood
Remove trim as needed. Cut out rotten jamb sections and splice in new wood with exterior glue and fasteners, or replace the full jamb if damage is wide.
Epoxy for limited pockets
After drying, wood hardener plus epoxy can rebuild small non-structural voids. Shape, sand, prime, and paint. This fails if the leak continues.
Seal and maintain
Reinstall flashing and caulk correctly. Slope the sill, keep storm doors adjusted, and repaint bare wood promptly.
For more repairs like this, start in the Rot & Outdoor Wood hub. If you are working through a related problem next, see Wood Decking Repair: How to Fix Cracked and Rotted Deck Boards or How to Fix Rotten Wood on the House: Exterior Siding Repair.
