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.


Red oak or white? Red Oak is porous and prone to rot and mildew.

There are at least wo kinds of wood bleach on the market. One is
oxalic acid, the other is a two-part bleach using sodium hydroxied
and hydrogen peroxide. I've tried both on red oak. The oxalic acid
didn't seem to do anything, the two-part bleach worked OK.

Shellac is reputed to be the best base coat to use as a moisture
barrier so that may be your best bet after you are down to bare wood.
For the best results it should be mixed fresh from flakes, see
http;//www.shellac.net. For second best results using pre-mixed
shellac check the date on the bottom of the can and use the freshest
you can find. If you use the 3 lb shellac from the can thin it
about 2:1 shellac to alcohol. Use dewaxed shellac.

Then you can go over the shellaced door with a good exterior
varnish.

Good luck.

--

FF