DIYbanter - View Single Post

wrote:

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

How to remove the black stains?

Oxalic acid (deck cleaner)...

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. Couple of possibilities are points of penetration of
hardware, a brittle former topcoat that fractured, a scratch that broke
the surface, a path under the topcoat around a door panel, ...