This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
The Phoenix kit requires that you sheet the wing with balsa using epoxy
as the glue. Aerotech also recommends that you vacuum-bag the wing for
the lightest wings possible. Vacuum bagging is a fairly new technique
that I will describe later.
The process involves preparing the wing skins, mixing the epoxy (need-
less to say, the 24 hour laminating variety, spreading it on the skins
with a squeegee, scraping most of it off, applying the skins to the
core, then assembling everything together in the core beds (the pieces
left over after the core is cut), and putting lots of weight on top
of the whole thing. Oh yeah.. the wing has to be kept straight so
you'd have to do this on a very flat surface. The more pressure you
can put on this, the better glue joint you'll have, and the less glue
you'll have to use, which makes for a lighter wing.
This is where the vacuum bagging comes in. The core bed/sheeting/core
assembly is put into a large bag which is sealed on all sides. Then the
air is pumped out of the bag. This is supposedly the equivalent of pi-
ling hundreds of pounds of weights on the core. In fact they tell you
to limit the vacuum to so many inches Hg otherwise the cores will crush.
Vacuum bagging is also useful if you are going to lay up fiberglass
on top of the balsa wing skins. Fiberglass cloth is now available in
very light weights and people often use it in lieu of a covering film
The way it used to be done was that the cloth was laid down and a thin-
ned (with alcohol) epoxy brushed into it. Then excess epoxy was removed
using rolls of toilet paper (discarding layers as they became saturated).
With vacuum bagging one lays down a sheet of drafting mylar on top of
the wet glass cloth, then puts the assembly in core beds. The assembly
is then vacuum-bagged. After curing the mylar sheets are removed and
you end up with a glass-like finish that is extremely light since all
excess epoxy has been squeezed out. This also obviates the need for
lots of the filling and sanding usually necessary before painting.