I have a wooden exterior door on my house (oak). I stained the
door
and then put an exterior polyurethane sealant on it (5 coats).
Moisture must have gotten under the sealant because some black
stains
have appeared under the sealant, to the point where it must be
redone.

I'm in the process of stripping the polyurethane and that's going
pretty well.

My questions a

Once the sealant is off, does the door have to be sanded down to
the
bare wood (remove all the natural oak stain that I originally
applied?

No, and it would be impossible w/ porous oak anyway unless you take
off
an inordinate amount of material

OK. thanks.


How to remove the black stains?

Oxalic acid (deck cleaner)...

I would apply it to the stains only, or the entire outside face.


What to reseal it with?

Good quality exterior spar varnish...


Don't say where the staining occurred so can't do more than
conjecture
on cause.

The stains were mostly on the lower half of the exterior, across the
entire bottom of the door, and near where a kickplate would go. The
door is covered by a portico so the top part is sheltered.

It did get a lot of exposure during the hurricanes last fall, so
seawater/rain/wind probably played a good part in all of it.
(We would have covered the door, but we weren't here.

Thanks for the tips.