Malcolm Hoar wrote:
I am currently refinishing a hardwood front entry door; oak,
I think.

I have just about completely removed the old poly and stain
by sanding. However, in a few areas, the surface wood is
just a tiny bit softer and more fibrous. I assume this is
due to some moisture penetration under the old poly.

These areas are still pretty solid -- can't dent with a
thumbnail. But I can see and feel a subtle difference when
sanding them.

What can I do with these areas prior to painting with fresh
(oil based) poly? I don't think sanding all of the slightly
soft material away is going to be practical.

I have seen various "wood hardener" products in the big box
stores but have never used them. Are they appropriate here
and do they work? Some other treatment?

Seems to me you're on the right track using a hardener treatment. One
that comes to mind is MInWax, part of their restoraation package with
the Bondo type filler. However, it is a resin + solvent system, and
the resin may or may not be hard enough for your purpose. Given that
situation, I would likely use a well thinned out oil based
polyurethane for maximum penetration and resand after thorough cure.
Take advantage of the spectrum of colored poly's to get the final
finish you want. Weathered oak may not repond very well to the usual
stains and fillers. Good luck.

Joe