How to remove the black stains?

Oxalic acid (deck cleaner)...

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


Try it...probably will want to do the whole door to even it out. If
is
effective against the particular stain (and I suspect it will be), it
will probably be fairly easy to get a reasonably good match.

Just wanted to say thanks to all who posted their responses.
The oxalic acid worked great and the door came back better than
I thought it would. I applied it to the entire door. It did lighten
the color somewhat, plus the sun bleached it over the past year,
so I do plan on restaining it before resealing.

Thanks again.

Stephen







...
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.

Probably a good plan/decision...

I'd say if it wasn't stained significantly prior to that time your
surmise is correct--the strong wind essentially did an accelerated
aging
test.