Sara says "Upon closer inspection, however, I discovered that they were
chewing the surface off of the wood and leaving hundreds of small tracks.
I tried spraying the wood with bee/hornet spray but this has not deterred
them."

Sara, these are almost certainly what we call "Bald Faced Hornets".  An inch
to 1 1/2" long, black, white and yellow body, and a white face.  Right?
These are also called paper wasps and build the huge cylindrical nests you
see in trees...most often after the leaves drop in the fall.  They are
literally gnawing strips of wood and the "tracks" you describe are where
they have taken a thin layer!  They masticate (chew) it, mix it with saliva,
and use it to expand their nest.

Good news...they are not going to do any real damage to your furniture;
except if you are fussy and don't like the streaks where they have
confiscated some surface teak.  Bad news...they are very aggressive, can and
do sting several times, and are easily provoked.  You bee/hornet spray is
not deterring them because it has a very short residual life.  It kills
quickly upon contact, but disintegrates very quickly.

My experience is that 2-3 individuals are doing the damage.  Kill them, by
hitting them with spray, and your problems will be solved (at least until
next year).  But be careful, because they will attack!

Lloyd
Lloyd Spear Owner, Ross Rounds(tm).  The finest in comb honey production.
http://www.rossrounds.com