No, it's not Sans Serif, it's Arial, just the bold-italic variant of
it. Try this:
M-: (face-font 'variable-pitch) RET
What do you see?
Correct, variable-pitch face is set to bold-italic variant of Arial; I get
"-outline-Arial-bold-italic-normal-sans-17-*-*-*-p-*-ascii-0"
I tried applying your patch but that was futile (I get the same result) as I cannot build emacs on Windows.
faces is preloaded, so it's not enough to recompile it. And w32 is
correct, since it should name the frame type (a.k.a. 'window-system')
value, not the system type.
You need to dump Emacs after byte-compiling faces.el.
Thanks for that info. Some one who can build emacs on Windows should be then able to verify the patch.