Polyurethane glue’s unique features
February 2012 71
Keith Sparks From the Build Table of ... | [email protected]
Below left: Polyurethane glue’s foaming
action tends to travel the path of least
resistance. Little clamp pressure is
needed to take advantage as it travels
into the wing to join the edges.
Below: Using tape to seal a seam will
force the glue to stay inside the model as
it foams. It works well on most lightly
sanded surfaces.
Left: In experiments, polyurethane glue
was resilient to every solvent the author
used. This makes cleanup difficult, so
keep paper towels around for cleaning
tools and fingers.
POLYURETHANE GLUE comes in
several brands and is found in nearly
every home-improvement store or craft
department. This adhesive has special
qualities and hazards not found in others,
so read the warning label. When exposed
to moisture in the air, the glue foams,
finding its way into every nook and
cranny of the material being bonded. This
makes it an excellent, lightweight bonding
agent for foam construction.
You could apply this glue with your
fingertip, but the glue will find its way
into every pore and gather dirt and dust as
it cures, giving you black fingers for the
next few days. The only way to remove it
is with a pumice stone.
Don’t bother trying solvents to clean
yourself; you will only remove your
natural skin oils, making the glue stick
better. I soaked cured polyurethane glue
balls in fairly volatile solvents for six
months and they came out unaffected,
aside from color. If you must use your
finger to apply or spread it, cutting a finger
from a rubber glove is your best choice.
Playing cards cut into strips and used
as if they were little putty knives work
well as applicators. For the large jobs,
foam-rubber blocks dipped in a shallow
pool of the glue can be used to dab it
along the edges of ribs or formers. You
can use the applicator tip on the bottle.
Remember to keep the tip and the cap on
the bottle clean and it should last for
several projects.
Polyurethane glue foams as it hardens.
After it is exposed to the environment,
the foaming and hardening begins.
Temperature and humidity play a major
role in the glue’s drying time and the
amount of foam it creates. A hot, humid
day in the workshop will cause a drop of
glue to foam twice as much as on a cold,
dry day.
A quick wipe with a damp cloth to the
surface is all that is needed to add
moisture, but little can be done to remove
it quickly. Drying time is similar to that of
wood glue and is a nonissue unless the job
is a large one. Gorilla offers quick-drying
polyurethane glue that works well on
smaller jobs.
When using polyurethane glue, the
trick is in controlling the foam. In most
cases we do not want the foamed glue to
reach the outer surface of the model and
blossom into a glue bud. Because the glue
follows the path of least resistance, all that
you need is a way to block it.
Sealing the edge with plastic box tape
works well. Masking tape is textured and
will not provide the seal needed.
If a glue bud does find its way to the
surface, it is best to deal with it before it
cures completely. This means checking on
the bond while you attend to another
building task. If a bud does appear and
has dried, your best choice for removal is
02sig3.QXD_00MSTRPG.QXD 12/16/11 1:52 PM Page 71to shave it off with a razor.
The foaming glue should not be
considered something to “deal with;” it
does enable us to form fillets in T-joints
such as what we find in fuselage
crutches or wing rib-to-wing skin joints.
The foam’s expansion also closes lessthan-
perfect bond joints.
Clamping sheet foam with tape or
using small weights are your best
options when working with
polyurethane glue. Straight pins can
work in a pinch; however, the glue
tends to stick to anything, including
straight pins. Removing the pin with a
twist should prevent the pin from taking
a chunk of foam with it when removed,
but not always.
Keeping a selection of small weights
about the size of a C-cell battery will
prevent a search for more during a
bond. The weights do not have to be
heavy, just evenly spaced; polyurethane
glue will not separate lightly clamped
parts when it foams.
Where weights are not feasible—
such as in fuselage crutches—masking
tape is the best clamping tool available.
Even if the sheeting is being forced to
conform to a curve, masking tape will
hold the shape while the glue cures and
the tension is easily adjusted.
As the glue foams, a plastic straw
can be used as a tool to spread the
foamed glue into a fillet by dragging it
through the foam, increasing the bond
area. Note that if you push the straw
through the foam it becomes a foamcollection
tool.
Nothing replaces epoxy for
important bonds such as motor mounts
or landing gear, but leaving the
lightweight work to polyurethane glue
will save weight on your model.
Edition: Model Aviation - 2012/02
Page Numbers: 71,72
Edition: Model Aviation - 2012/02
Page Numbers: 71,72
Polyurethane glue’s unique features
February 2012 71
Keith Sparks From the Build Table of ... | [email protected]
Below left: Polyurethane glue’s foaming
action tends to travel the path of least
resistance. Little clamp pressure is
needed to take advantage as it travels
into the wing to join the edges.
Below: Using tape to seal a seam will
force the glue to stay inside the model as
it foams. It works well on most lightly
sanded surfaces.
Left: In experiments, polyurethane glue
was resilient to every solvent the author
used. This makes cleanup difficult, so
keep paper towels around for cleaning
tools and fingers.
POLYURETHANE GLUE comes in
several brands and is found in nearly
every home-improvement store or craft
department. This adhesive has special
qualities and hazards not found in others,
so read the warning label. When exposed
to moisture in the air, the glue foams,
finding its way into every nook and
cranny of the material being bonded. This
makes it an excellent, lightweight bonding
agent for foam construction.
You could apply this glue with your
fingertip, but the glue will find its way
into every pore and gather dirt and dust as
it cures, giving you black fingers for the
next few days. The only way to remove it
is with a pumice stone.
Don’t bother trying solvents to clean
yourself; you will only remove your
natural skin oils, making the glue stick
better. I soaked cured polyurethane glue
balls in fairly volatile solvents for six
months and they came out unaffected,
aside from color. If you must use your
finger to apply or spread it, cutting a finger
from a rubber glove is your best choice.
Playing cards cut into strips and used
as if they were little putty knives work
well as applicators. For the large jobs,
foam-rubber blocks dipped in a shallow
pool of the glue can be used to dab it
along the edges of ribs or formers. You
can use the applicator tip on the bottle.
Remember to keep the tip and the cap on
the bottle clean and it should last for
several projects.
Polyurethane glue foams as it hardens.
After it is exposed to the environment,
the foaming and hardening begins.
Temperature and humidity play a major
role in the glue’s drying time and the
amount of foam it creates. A hot, humid
day in the workshop will cause a drop of
glue to foam twice as much as on a cold,
dry day.
A quick wipe with a damp cloth to the
surface is all that is needed to add
moisture, but little can be done to remove
it quickly. Drying time is similar to that of
wood glue and is a nonissue unless the job
is a large one. Gorilla offers quick-drying
polyurethane glue that works well on
smaller jobs.
When using polyurethane glue, the
trick is in controlling the foam. In most
cases we do not want the foamed glue to
reach the outer surface of the model and
blossom into a glue bud. Because the glue
follows the path of least resistance, all that
you need is a way to block it.
Sealing the edge with plastic box tape
works well. Masking tape is textured and
will not provide the seal needed.
If a glue bud does find its way to the
surface, it is best to deal with it before it
cures completely. This means checking on
the bond while you attend to another
building task. If a bud does appear and
has dried, your best choice for removal is
02sig3.QXD_00MSTRPG.QXD 12/16/11 1:52 PM Page 71to shave it off with a razor.
The foaming glue should not be
considered something to “deal with;” it
does enable us to form fillets in T-joints
such as what we find in fuselage
crutches or wing rib-to-wing skin joints.
The foam’s expansion also closes lessthan-
perfect bond joints.
Clamping sheet foam with tape or
using small weights are your best
options when working with
polyurethane glue. Straight pins can
work in a pinch; however, the glue
tends to stick to anything, including
straight pins. Removing the pin with a
twist should prevent the pin from taking
a chunk of foam with it when removed,
but not always.
Keeping a selection of small weights
about the size of a C-cell battery will
prevent a search for more during a
bond. The weights do not have to be
heavy, just evenly spaced; polyurethane
glue will not separate lightly clamped
parts when it foams.
Where weights are not feasible—
such as in fuselage crutches—masking
tape is the best clamping tool available.
Even if the sheeting is being forced to
conform to a curve, masking tape will
hold the shape while the glue cures and
the tension is easily adjusted.
As the glue foams, a plastic straw
can be used as a tool to spread the
foamed glue into a fillet by dragging it
through the foam, increasing the bond
area. Note that if you push the straw
through the foam it becomes a foamcollection
tool.
Nothing replaces epoxy for
important bonds such as motor mounts
or landing gear, but leaving the
lightweight work to polyurethane glue
will save weight on your model.