I'm unsure if there is a perminate fix short of replacement. If the repair area moves any at all,filler will loose grip on the wood. To remove rotted wood look at the burs in the thread below asking "What kind of bits". If you go with filler,use one with hardener like Bondo. Don't atempt to apply filler in a single application because it's a bear to level out. Build layers useing an assortment of putty knives. There is a auto body shop rasp that works well in leveling partly cured filler,much faster than sandpaper. Once you have done your best with filler,use thin filler that comes in a tube like tooth paste to fill imprefections. You will of course need to sand the final repair.

I'ld replace that entire board ... looks to be a 1"x6" with a rounded over edge. Get it in a pre-treated pine variety of the appropriate length and paint it white to match, and fasten it securely to the wall.

That looks like moisture damage, most likely from either warm air coming in through the cracks and leaving condensed water on your wood, or cold air coming in through the cracks and condensing the moisture in the house where it ends up on the wood. Either way it's bad for your wood.

After you have replaced the board properly, you will definitely want to look in to replacing the weatherstripping and seals around that area to solve the actual cause of this problem, or you will end up replacing the board again in the future. Pre-treated lumber will stand up to that kind of abuse for longer than untreated wood, but in the end, the water will win.

Not repairable; the jamb side needs to be replaced. The jambs will have to come out. Unless the door is solid hardwood, it's cheaper to buy a new steel prehung unit. Get one with a composite frame or this will need to be dona again in (how old is the door?) The wood jambs on new doors and windows are made from the worst, crappiest pulpwood. No rot-resistance at all.
Casey

Hi Skie_M,

Thanks for your post. Two questions:

1. How can I fasten the pine variety to the wall? Glue or nail?

2. Can I paint it after I fasten it to the wall?

Thanks!

eibc, Skie is incorrect it is a pc of door jamb with a rabbetet groove for a bulb weatherstrip. The door certainly appears to be a steel door. A new jamb leg would need to be routed and daddo'd exactly the same for it to work as a replacement. As Casey alluded to, it will be much easier and probably not much more expensive to purchase a new prehung unit and replace the entire thing.

None of the solutions offered will work long term without figuring out and building proper flashing details. A new door without them will rot just like the old.

That too, but thought that went without saying Treb. And your right before you say it, I made an assumption..........