Window Glazing
The window sash shown in this
process are originals from an 1850 house.  
They are all nine light sash, with 12x14 inch
panes.  All windows in this house are double
hung-a bit unusual for this time period.  

They needed complete repair, so we took all
the sash out and have plywood in the jambs
until the sash are ready to go back in.

With hundreds of feet of glazing to do, an
easel was quickly built out of 2x4s and scrap
lumber, so that all glazing could be worked
on in the horizontal position with a turn of the
sash.  One side of the easel provides a
comfortable position to work on two levels,
while the other side provides a position to
work on the third level, without any bending
over.
There are a fair number of different brands, and types of
glazing available.  Original window glazing was simply
linseed oil mixed with chalk.  It lasts a few years.  

We want to do the best we can for longevity, while still
leaving it easy to replace a window pane when necessary.  
Some glazings set up so hard that it becomes difficult to get
the hard glazing out, when a window pane needs replacing,
without damaging an adjacent pane while getting the hard
glazing from around the broken pane.  We will avoid using
some of the modern, extra tenacious glazing for old sash.

I started using DAP glazing in caulking tubes soon after it
came out.  It stays soft enough to remove three decades
later, and is at least ten times as fast putting it in place with
a caulking gun compared to hand applying glazing with a
putty knife.  It has very good tenacity in bond to primed
wood, and glass, but has one problem.  It shrinks, even
though there is a claim that it doesn't.  

This is not a glazing to use if you want to paint it tomorrow,
or even next week.  As the picture above shows, we put it on
thicker than we want it to start with, let it dry for several
weeks until it stops shrinking, and then trim it to final level.
is finished.  The chisel is a mid 20th Century Stanley carpentry
chisel ground and sharpened to a 20 degree bevel.  Modern
woodworking chisels, with fancier steel, cannot be sharpened to
such a thin bevel.  The chisel is sharpened way past the
sharpness of a razor blade (see my sharpening video on
youtube                                                  ), and very easily slices
the glazing.

This process takes longer from start to finish of the job, but actual
time on any given sash is several factors faster than the old way,
even though it's a two step process.  The same easel will be used
for painting.
The picture above shows one section of glazing after trimming, now
ready for paint.  There will be some dips and imperfections here and
there, and a quick setting glazing   Aqua Glaze  will be used to level
out those few places, allowing it to be painted without more long
waiting time.
With single pane windows, there will be some conditions where water
will condense on the inside of the window panes, especiall in these
museum houses that don't always have heat and air conditioning
inside.  

There will sometimes be enough of this water to run down onto the
wooden parts of the sash.  For this reason, don't use interior paint, if
the sash are painted on the inside (sometimes 18th and 19th
Century sash are not painted on the interior).  Use exterior paint
which is engineered to handle direct water contact.

My personal favorite for painting sash is currently Sherwin-Williams
Emerald.  Gloss is used on the outside for ease of cleaning, and
usually Flat on the inside to match the old paint.  The Gloss on the
outside doesn't stay glossy but for a short time when it's new, and
after cleaning, but I've found it's worth living with the high gloss for
thes short periods.
When we install replacement panes, or reset old ones, I like to bed
them to the inside surface.  There are modern caulkings designed
for just this purpose, but I find them too tenacious for easy
replacement of a broken pane.  We just need a cushion to make
against movement from thunder, or ground shaking for whatever
reason.  The other purpose of this bedding is to help keep out the
condensed water whenever it appears on the inside of the panes.

I use latex, or acryillic caulking for the interior bedding.  This is the
cheap stuff without silicone in it.  Latex caulking with added silicone
does not clean off of the glass as easily when we trim it.  The
cheap latex caulk does not have the best bond strength, but that's
fine here where we just want it to stay soft, and take up space.

It will be trimmed on the inside, very similar to the way the outside
glazing is trimmed, but on the inside we don't want it to show at all.  
It will be trimmed in line with the wood, and covered with a layer of
paint that covers this inside gasket, and the paint bonds to the
glass on the inside.  This keeps water out from behind the glass,
and helps the outside glazing get the most longevity.

The picture above shows the squeezeout of this bedding.
the wood with a thin, single edged razor blade.  The chisel can't be
used here, or the thicker bevel can pull some of the caulking up
from behind the wood.
The super sharp chisel we use for trimming the glazing works best
for final trimming of the bedding off the glass, once it's been
separated from the wood by the razor blade.  A lot of times, on these
old sash, there will be some erosion on the inside molding profiles of
the sash by the condensed water running down onto it.  Any extra
indentations still left after the bedding caulking has been trimmed will
need to be filled, and leveled, before painting.