In my 100 yr old unfinished attic the wood sash or window is in dire need of
replacement (quite disappointed over the built to last qualityG). My
attempted repair turned into a replacement(rotten wood). Anyway I'm building
the new wood sash with milled 1 1/4" fir with bridle joints and expect to
glaze the window with traditional putty

Should I prime and/or paint the fillet or under the glass? Somewhere or
sometime I thought I read not to finish the fillet although I have no idea
as to the why not. The existing window, as are all of the others in the
house are bare wood albeit all are mostly original.

The existing window is 66" by 17" with 2 muntins...will I burn in "old house
hell" if I use a larger single pane and use 2 surface mounted dividers? The
attic is not readily accessible and from the street I don't think it would
be noticeable.

Thanks Rod