Glue Size (from the book "Furniture Finishing Textbook" by W. E. Martin)

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SIZING BEFORE SANDING

Since wood is a fibrous material, there is some tendency (depending on
the wood and its moisture content) for surface fibers to tear out
rather than to cut cleanly during sanding. Also, surface fibers may
absorb atmospheric moisture and cause re-sweIIing after sanding, thus
raising a fuzz in the surface that will affect the clarity of
subsequent staining. This fuzz problem is subdued by sizing just ahead
of the last belt sanding operation. The sizing acts as a stiffener on
the surface fibers, permitting them to cut clean and to remain smooth
until the finishing operation is started.

Glue-size made with animal glue is the most widely used. It is
commonly made by adding water to "pot glue" used in assembly, usually
in the proportion of 1 part pot glue to 8 to 12 parts water.
Proprietary sizing compounds based on waterdispersed resins are
occasionally used. Size may be applied by spraying, brushing, sponging
or roll coating. Spraying is fast and economical and is improved by
heating. Warm spraying of glue size at around 12S'F. is common
practice. Many finishers think that brushing or sponging does a better
job, but unless care is taken in the application, this method can
produce a streaked or spotty effect that will show up in the
subsequent staining. Roll coat application of size on flat panel stock
is entirely practical. It is done with the same type of roll-coat glue
spreader used for coating cross banding when laying up veneers.

In order to be effective, size must be thoroughly dried so that fibers
are in a stiffened condition before sanding. The drying time will
depend on the amount of size applied and the atmospheric moisture
conditions. It can be speeded up by oven drying at moderate
temperature, for example, one hour at 120¡F.

When sizing is used, it is generally applied between the initial and
final belt sanding operation. Most of the size is removed in the final
sanding. Too much size remaining in the surface will interfere with
staining, causing an uneven, blotchy appearance, especially when using
non-grain-raising stain.