"Andy Dingley" wrote in message
I have some oak doors (external) that are approaching 100 years old and
are
in remarkably good fettle.

That's because they're oak. If they're English white oak (or French)
then they'll last well just thrown outdoors and ignored, so long as
they're a big enough cross-section that radial cracking isn't a
structural problem.

Tht was my reaction. I've seen centuries old oak doors, untreated and in
daily use, which are fine.

Mary