no, dammit, not the painter) said:

I have what was sold as Honduran mahogany

to me. What is bothersome is that parts of my boards ...
just look fuzzy.

So, it this the behavior of some wood other than HM? Or is
it sapwood? Or is it my bad finishing technique? ?? ??

There is a wood condition, I have forgotten the obvious name. Anyway,
there is nothing you can do, it will not go away, you can't sand,
scrape or plane it out.

I found this reference in Brazilian Mahogany:
http://www.timpan.co.nz/timbers/mahogany_swietenia.html

"Fairly easy to work. The presence of tension wood gives rise to
fuzzy surfaces and deeply interlocked grain causes some grain tearing
in quarter sawn surfaces."
--
McQualude