This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
This is an art in itself. You will need some "Japanese" tissue
(from SIG or Peck Polymers) and some clear low shrink dope. I
have found that SIG Nitrate dope is less likely to warp the
wings, and fills the pores faster. The tissue comes in
assorted colors to decorate your model. Use 2 colors, with a
darker color on the bottom, for visibility in the air, and a
lighter color on top for visibility on the ground. Green is a
poor choice for the top, but Blue surprisingly looks pretty
dark in the sky. A couple primer coats of dope are applied to
the balsa surfaces. Another coat is used to stick the tissue
down to the balsa. More coats over the tissue soak through and
bond the tissue to the balsa, and fill in the pores.
Brett Buck describes Tissuing Glider Wings in a bit more
detail: "For MR sized models, Japanese tissue is probably the
best commonly available choice. It's available from
Peck-Polymers, and many other sources.
To apply, put several coats of just-thinned-enough-to-brush
clear dope, Use SIG Nitrate, Brodak, SIG Lite-Coat, or in a
pinch, Aero-gloss. DO NOT use SIG Supercoat - it shrinks too
much. Let it dry 4-8 hours between coats, unless you're using
Brodak (you only need to wait ~2 hours). When it starts
getting getting slightly shiny, that's enough.
Cut the tissue slightly larger than the area you want to
cover. The tissue has a grain to it. You can see it, but you
will also note that it tears easily along the grain but with
great difficulty across the grain. The grain should run along
the long ways of the pieces, in this case spanwise. The tissue
also has a top and bottom. One side is very shiny and hard
looking and the other side is matte finished. Use the tissue
Lay it down and smooth it out. Use it dry. Start in the middle
of the area, and brush a very thin mix of dope and thinner
(mostly thinner) from the middle out. It will run through the
tissue and soften the underlying layers, sticking the tissue
down. Smooth it out as you go, and if there's a wrinkle, put
on a lot of thinner to loosen it up, and then lift the tissue
and pull out the wrinkle. It's very strong, and difficult to
tear, so you don't have to be too gentle. Once the whole area
is covered, I like to go back over the entire surface with
more thinned dope, let it sit for a few seconds, and then rub
it down with a paper towel to really force the tissue into
intimate contact with the wood.
If you need to go around a compound curve, wet the tissue with
water just in that area, and pull away to stretch it around
Once you are satisfied with the covering, go around the edges
of the piece with 220 sandpaper to sand into the tissue to cut
off the excess. This is better than cutting it with a knife,
since the slightly fuzzy edges stick down better. Then seal
the edges with more dope. Then flip it over and repeat for the
other side of the wing.
Once the whole thing is covered on both sides, put on a few
coats of 50-50 dope. It will get more and more transparent for
the first few coats. 2-3 should be sufficient, but 4-5 will
make it very solid and can be rubbed out if you choose.
If the wing warps due to shrinkage, it can be straightened
right after a coat of dope when it's soft. Once everything is
dry, heat or steam will be required. Beware of heat guns, as
dope (particularly nitrate) is highly flammable. For most thin
glider wings, steam will work. For thicker wings (like 1/4"
balsa") steam will not work. In this case, wrap the wing with
a bath towel and then pour boiling water on the towel until
it's soaked, let it sit for a few seconds, then twist against
the warp for a few minutes. Then take off the towel. I had to
do this just last week with my Ecee Thunder wings.
I'm sure others will add to anything I've missed. This is the
traditional way of finishing any balsa part. If you want a
good finish, this method saves a tremendous about of weight,
because trying to fill bare wood grain with filler takes a lot
more filler than than trying to fill tiny holes in tissue. Not
to mention that the filler inevitably shrinks down into the
grain after a while. It also adds a tremendous amount of
strength compared to the negligible weight gain.
An alternative material that I prefer in a lot of cases is .2
oz/square yard graphite matte. This is available from
Aerospace composites. Ask for "soft" matte vice "hard"
matte. Soft fills more easily. Apply with dope in the same way
(except for the water for compound curves). It goes around
compound curves much more easily than tissue, and is much
stronger. It's about the same weight once it's applied.
For larger models, replacing the jap tissue with various
grades of silkspan is preferred. This is particularly true if
you have large open areas. It's tougher and heavier than jap
tissue and is applied wet. For almost any rocket purpose, OO
silkspan is plenty enough, but there are heavier grades GM
(Gas Model) or SGM (Super Gas Model). SGM is pretty tough
stuff. All tissue-type products add tremendous strength to the
part. Iron-on or synthetic coverings like Monokote, Solarfilm,
Polyspan (polyester tissue) are all very soft and will not
help the rigidity very much at all."