The masked hard edge and freehand soft edge are evident on the nose of the M. 20.
Polyspan and paint
by Paul Kohlmann
Last month we tackled the last of the major woodworking topics required to frame up our
Miles M. 20. Now that the framework
is complete, it’s time to talk about
Previous “Construction Series”
articles have focused on concepts
and techniques geared toward the
first-time builder. This month, we will
take on more advanced material.
This isn’t completely intentional—
it’s just that these are the techniques
that I chose to use on the prototype
M. 20 before it became the subject of
Types of Covering
The classic covering material for small
stick-framed models is tissue paper
applied and sealed with dope. Larger
models would be covered with silk
and dope. When done well, tissue-covered or silk-covered
aircraft are beautiful and their coverings are easy to paint.
Some negatives are that these coverings can be challenging to
apply properly. Dope also contains nasty solvents and is now
relatively hard to find.
Heat-shrink film coverings such as MonoKote are much
more popular today. Dope isn’t needed because the adhesive
is already coated onto the thin polyester film. Just stretch it
over the framework and iron it on (see “Covering Techniques,”
February 2014). These coverings come in many colors and
several weights. I use them often, but when it comes to Scale
warbirds I have two complaints: they tend to wrinkle in the
sun and it can be difficult to get paint to stick to them.
That’s where Polyspan comes in. Polyspan is a hybrid. Like
MonoKote, Polyspan is made from polyester and it shrinks
with heat. But like tissue, it is woven from fibers and it is easy
to paint. Polyspan is applied by painting the framework with
an adhesive in the same way as tissue or silk. The result is a
lightweight covering that is resistant to tears and punctures.
I chose to cover my M. 20 with Polyspan so that I can
show this model on sunny days without worrying about it
Builders have been using Polyspan as a covering material for
quite some time. Early adopters simply applied the same dope
and techniques they had used on silk or tissue. This works well,
but as I mentioned, dope contains harmful chemicals and it
isn’t as readily available as it once was.
Other builders have experimented with friendlier
chemistries to replace dope. The goal of these experiments is to
identify water-based adhesives and sealants that can be found at
a hardware or craft store.
The process used on the M. 20 was developed by two of
my online friends. Jim Arrington and Steve “Fuzz” St. Martin
have both achieved beautiful results using Mod Podge as the
adhesive and water-based polyurethane as the sealer. Their
techniques will be the subject of this segment.
29 Model Aviation OC TOBER 2015 www.ModelAviation.com