Fairly new exterior door, already starting to rot at bottom.

I have a side entrance to my garage, and we put on new door on it not too long ago - but could have been a couple of years - not sure. I was just sweeping the area and wiping off dirt from the door and noticed the bottom -it looked discolored, so I poked my fingernail in it and sure enough it's soft. I actually thought this was a fiberglass door but maybe it has a wood center - I guess it does. Anyway, it came pre-primed, and I have never gotten around to painting it completely. Just didn't make it a priority and forgot about it. So maybe I was a dumbass for not doing that. Anyway, I don't know why it's getting so wet down at the bottom, but clearly it is. The old door ( the frame had had a lot of rot at the bottom that we had repaired a couple of years back by replacing the bottom section with composite framing) was kicked and damaged by a drunk college student so I had to replace the entire thing. So obviously there is a dampness issue going on there, but I never ever see standing water there. We did have a crap TON of rain with Hurricane Matthew but I was not home until after it was over and I had no damage - water never made it up my driveway. But maybe this this side door gets pelted with rain - I'm really not sure. There is a big willow oak about 25 feet outside that door, so there is some protection from direct sun and "sideways" rain I would think.

So now that I have discovered this, what is the best way to address it to make sure that the bottom does not deteriorate further? Anything I can do from the outside to prevent water seepage?

 

The door should be taken off its hinges and laid on its side or on a pair of sawhorses. You should remove the door bottom weatherstripping and seal the bottom of the door. There is obviously some bare wood that is sucking up water like a wick. You may need to sand the edge of the door fown, prime it with a oil primer (like Kilz original) the repaint it. Same goes for the door bottom once you remove the weatherstripping. Then once it is painted, but before you put the door bottom weatherstrip back on, run a bead of clear 100% silicone on the exterior edge of the door bottom, then press the door bottom into the sealant as you put is back on. That way you won't have water getting trapped between the wood and the weatherstripping.

Additionally you could consider putting a storm door on the door. That's the he whole purpose of a storm door... to protect your primary door from the wind and rain. If you don't have gutters above the door, you might consider that too.

 

I'm guessing you did not clean and paint the door and jambs with two coats of Acrylic latex paint as the directions that came with the door stated.
I see the paper shims left in place on the door and jamb that should have been removed as a tip off.
Most of those doors are sold primed but not painted.
Unless that's PVC brick molding it also should never have been installed tight to that stoop, waters going to get trapped under it and rot it out.

 

The factory primer on the wood portion of the door isn't much of a primer. I 2nd X's suggestion of using an oil base primer. Oil primers do a better job of sealing the wood so moisture can't penetrate. I also like the recommendation for installing a storm door.