BUILDING BOARD – Covering R/C Airplanes with Plastic Film
By Bob Crawford
This article is a compilation of 5
articles that appeared in the GCRCC Flightline between July
1997 and March 1998. The five topics are the following:
The Stretch Rule, Part 1
The Stretch Rule, Part 2
Using the Trim Iron
The articles were originally aimed at
beginners to the hobby who were faced with applying covering
for the first time to a trainer. These days, almost all
newcomers start with ARF trainers, and those who continue with
ARF’s will have little need to learn covering techniques.
For the subgroup of enthusiasts who decide to do their own
building, there have been numerous how-to books and articles
published about covering with film. I remember reading
everything I could find when I started covering, and I still
had to learn and develop many of my own techniques, some of
which may be contrary to “conventional wisdom.” I hope
this article will afford an additional “head start” to
One disclaimer is necessary. The only
film covering I use is Ultracote (aka Oracover). I am sure
that the general techniques described here are applicable to
most covering films, but it is certainly possible that some
modifications will be necessary when using other materials.
Everyone knows that a paint job on
anything, whether a model airplane or a house, is only as good
as the preparation job on the underlying surface. The same is
true for covering models with heat shrink film. In fact, there
are probably fewer ways to get around poor surface preparation
with film than there are with paint.
Obviously, surface preparation is done
with sandpaper, so let's talk about this essential tool. The
brand of sandpaper product you use is a matter or personal
preference. The various sandpapers available are all different
routes to the same end. All I can say here is that I've
settled on 3M garnet sandpapers as my personal favorite. This
paper needs changing fairly frequently, but I like the way it
performs so well I'm willing to live with this deficiency. I
also like the aluminum T- bars for sanding blocks, but each
builder will settle on a product that feels most comfortable.
You just have to start trying different things and pick the
one you like best.
The sandpaper grits I use are 60, 100,
and 220. I also use 3M silicon carbide 400 grit paper for
final finishing (more on that later). The 60 grit paper is
used mainly for hardwoods, and for large balsa blocks
(wingtips, cowls, etc.) that need some fast persuasion to get
close to their final shape.
The 100 grit paper is the real workhorse
in building balsa models. I use it for every aspect of final
shaping, as well as smoothing sheeting joints, and even
smoothing the surface of parts that are already cut to shape.
When I have a model ready for covering, there won't be a
square inch on it that hasn't been touched with 100 grit paper
at some point.
With 220 grit paper, the objective is not
shaping but finishing. Here, we're removing the roughness left
by the 100 grit paper. Once again, I will go over the entire
model with 220 to prepare for covering. When sanding with 220
on flat surfaces, I usually use the sanding bar. With curved
surfaces, however, I just use my hand to back up the
sandpaper. For example, I'll hand sand wing leading edges,
wingtips, fuselage edges, tail feather edges, contoured cowls,
etc. If you recoil at the idea of hand sanding, simply glue
the sandpaper to a piece of foam instead.
I mentioned earlier the use of 400 grit
paper for final finishing. I'll admit that 220 grit sanding is
probably perfectly adequate for film covering. It's just that
I'm a bit fussier than necessary (you already know that, don't
you?) so I like to go over everything with 400 for that extra
degree of smoothness. Once again, It's a personal choice.
The final procedure in covering
preparation is balsa dust removal. Here I've developed a
method that's a bit different from usual practices. Most
builders vacuum the wood, or clean it with a tack rag (or
both). In my experience, a little more aggression is needed to
really get all the dust out. I think dust removal is extremely
important. Every microscopic area of covering adhesive that's
bonded to dust is not bonded to the model.
My principal tool for dust removal is a
roll of Scott Shop Towels, which are blue in color. The
technique is to use the paper towels as if they were a 10000
grit sandpaper, rubbing all wood surfaces with a vigorous
swirling motion, and using even more downward pressure than
you'd use with sandpaper. The advantage of the blue towels is
that the beige balsa dust being removed shows up very clearly
against the blue background. I keep rubbing the wood with
fresh surfaces of towel until no more dust is visible. When
this happens, the surface is truly dust free. Everyone knows
how smooth sanded balsa feels, but when you remove all the
dust, it actually has a sticky feeling to the touch.
At this point, the model is ready for
covering. As I work on each piece of the model, I will cut the
piece of covering from the roll, and just before removing the
backing I'll give the piece to be covered a quick rub-down
with my bare hand to brush off any dust that may have settled
from the air. Then, peel the backing and get the covering
ironed down as soon as possible.
Irons: Most of my own covering work has been done with one
of the basic "15 dollar" covering irons. This iron
works fine, and I've never had any complaints about it. I
always keep a hot sock on my iron, and replace it when it
begins to look "toasted." More recently, I decided
to treat myself to the 21st Century iron, which costs in the
$30 range. I've seen two principal benefits from the extra
cost. First, the 21st Century iron has rounded edges. The
sharp edges of the standard iron can create grooves in the
covering if you're not careful. Second, the control dial is
calibrated in degrees, so you can set the desired temperature
directly. Most of the time, I set this iron in the 300-325
degree F range for use with Ultracote. On the "el
cheapo" irons, the dial has arbitrary numbers, so you
need to buy a covering iron thermometer and calibrate the dial
yourself. On a more subjective level, the 21st Century iron is
lighter, and has a better balance and "feel." You'll
have to decide if these features are worth the extra money.
Irons: The Top Flite trim iron is a tool I consider
absolutely essential in covering. I've found that a lot of
newcomers to the hobby haven't been informed about the
usefulness of this tool, and don't bother to acquire one. On
many occasions I've been asked how it's possible to apply
covering to complex rounded surfaces such as wing and tail
feather tips without wrinkles. The answer is the trim iron.
Although the areas of a model that require the trim iron are
small, I probably spend more time with this tool in my hand
than the main sealing iron. Use of the trim iron is discussed
in Part 5 of this article.
I mentioned above that I had switched to
the 21st Century covering iron. When 21st
Century came out with their own trim iron, I was excited
enough to run out and buy one immediately. What a mistake! It
turns out that this trim iron has a maximum temperature around
300 degrees F, which is far lower than the hot setting on the
Top Flite iron. The 21st Century tool is completely
useless for the techniques described in Part 5, and for me was
a waste of money.
Guns: Some modelers use the heat gun very extensively in
applying covering. I prefer to use the iron for solid
surfaces, and only use the heat gun to shrink covering over
open wing bays. Another less obvious use of the heat gun is
removing covering. If you have to strip off covering to repair
a crashed plane, blast it with full heat from the gun and it
will peel right off. If you try to do it cold, you'll probably
pull off chunks of balsa and create a bigger mess than you
Tools: I probably do 99% of my trimming using #11 blades
mounted in a standard 5/16" handle. And 99% of that uses
only the pointed tip of the blade. I've estimated that the tip
of a new #11 blade will effectively cut only 10 to 20 inches
of covering before it is dull enough to be replaced. As a
result, I normally use 30 or more blades to cover an entire
model. This isn't as wasteful as it sounds, since blades used
for trimming covering are essentially equivalent to new,
unused blades for any other cutting application. My spent
covering blades are recycled to a box on my building table for
use in cutting balsa and everything else that requires #11
Using a sharp, new blade is especially
critical when you're trimming covering that overlaps another
section of covering, and you only want to cut the top layer.
This is another topic that will be discussed later.
Razor blades are also useful, but I have
found no use whatsoever for single-edge razor blades in
covering. These blades are great for housepainters to use in
their scrapers, but, in my opinion, they are just not sharp
enough for use with plastic film. When I need a razor blade, I
use the double edge blades (preferably stainless steel) used
for shaving. For obvious reasons these must be handled with
great care. You can break them in half and use the edges
separately, or cover one edge with tape for protection.
Tape: Perhaps my favorite covering tool is one that most
modelers don't seem to be aware of yet. It is 3M Scotch brand
2070 "Safe Release" masking tape. This stuff looks
like ordinary 3/4" masking tape, but has the same
adhesive used in Post-It notes. If you've ever used regular
masking tape on covering, you probably found it will pull the
covering right off the wood when you remove it. Not so with
the Safe Release tape. It sticks to film covering very
securely, but peels off as easily as Post-Its come off paper.
The number of uses I've found for this
tape could probably fill a whole article. To mention a few, I
always use it as a blade guide for making long trim cuts that
I want to come out straight. Use it to pre-align stripes and
other decorations to eliminate the "eyeball" and
guesswork factor. Use the tape as a marker when you need to
create an even 1/8” overlap with two pieces of covering.
This product is so useful in covering, I think it should be
available in every hobby shop and catalog. Until more modelers
discover it, though, you'll have to go to elsewhere to get it.
It is available in different widths, but I like the ¾”
best. I’ve had luck finding it at the Michael’s art supply
Tools: It's important to own a good quality scissors (e.g.
Fiskars). When making trim pieces, you'll be cutting covering
from the roll with a knife on some sort of cutting surface
(and you can use the "used" #11 blades for this).
You need a variety of metal straightedges ranging in length
from 6" to 36." Some people use glass as a cutting
surface. I prefer to use Plexiglass sheets, which are more
expensive than glass, but not nearly as expensive as the
"self-healing" cutting mats you can get. I
occasionally use the Top Flite Smartstripe device for cutting
really long stripes, but find a straightedge and knife to be
adequate for shorter ones. I once tried to use the Top Flite
Smartcut trimming tool, but decided the amount of practice I'd
need to learn to use the thing would probably waste a whole
roll of covering.
This is a fundamental principle of
plastic film covering that I've decided to call the
"Stretch Rule." This is just a name I made up for
the convenience of this article. The stretch rule itself is
nothing I can claim credit for. I first learned about it while
watching a video about covering techniques. I think most
modelers learn it on their own through experience. Stated as
simply as possible, the stretch rule is this: Plastic
film covering must always
be stretched at the same time it is being attached to a model
using a heat source. The bottom-line message here is that
the techniques you use in covering, and how the job turns out,
will depend a great deal on how literally you interpret the
In order to illustrate the stretch rule,
let's imagine a simplest possible case. We're going to cover a
horizontal stabilizer for a typical 40-size trainer. The
stabilizer has been cut from 1/4" sheet balsa, the
trailing edge is straight, the leading edges are straight but
tapered back, and the tips are rounded. The leading edge and
tips have been sanded to a rounded cross section. The
stabilizer is not attached to the fuselage, and we'll cover
the right and left sides separately, leaving the center
fuselage connection uncovered. We'll cover the bottom first,
since this makes the edge seams for the top covering less
Let's cover the bottom left side of the
stabilizer with total disregard for the stretch rule. Lay the
stabilizer bottom-side up on your work table, cut a piece of
film large enough to cover it, peel the backing and position
the film on the part. Now, with one hand held behind your back
or in your pocket, iron the covering down using your other
hand. Since the part we're covering is perfectly flat, there's
a good chance this "one-handed" technique will turn
out OK. There's also a good chance that a few wrinkles will
appear during the ironing process, in which case you'll need
to "mash" the wrinkles down using heavy iron
pressure. In that case, you'll be left with telltale ridge
lines that show where the wrinkles were.
Now let's cover the right bottom using
the stretch rule. Set-up is the same as before, but this time
we'll use both hands. Also, be sure there is at least 1"
of excess covering extending beyond the edges. Begin by
ironing down about 1/2" of covering chordwise along the
fuselage joint (this can be done with one hand). Next, grasp
the excess covering at the stabilizer tip with your thumb and
index finger, and using the other three fingers as a lever
point on the stabilizer tip, stretch the covering firmly while you iron down a wide swath
spanwise working from root to tip. Finally, use the same
procedure along the leading and trailing edges, stretching the
covering while you finish sealing it to the wood. Move the
stretch points along as you iron, so that the section being
sealed is always under stretching tension. This time there
shouldn't be a hint of a wrinkle.
Imagine what would have happened if this
stabilizer were a bit thicker, and had been sanded to an
airfoil cross section. In this case, the non-stretch procedure
would likely have generated a complex pattern of wrinkles, and
squashing them out would have produced a rather unsightly job.
With the stretch rule method, however, the airfoil shape would
have been irrelevant to the covering procedure or the
Getting back to our stabilizer job, next
we have to iron down the edges. When doing edges and seams,
many modelers use a technique I call "pre-trimming."
This simply means that you leave approximately 1/4" to
1/8" of the covering unsealed adjacent to the edge or
seam, trim off the excess film, and then iron down that last
1/4" or 1/8." There are several tools you can use
for the trimming: a razor blade, the cutting edge of an Xacto
blade (rather than the tip), or even a scissors, whichever you
prefer. Pre-trimming is necessarily
a violation of the stretch rule. This should be obvious,
since you've removed the excess covering needed to do
Let's use pre-trimming on the stabilizer
left half, to maintain consistency with our stretch rule
violation. Trim the excess so the edge will come out at the
50% point of the stabilizer's thickness, and iron down the
edges. The leading and trailing edges are straight lines, so
they should iron down smoothly. When we get to the tip,
however, we encounter a compound curve. There is simply no way
to iron down this pre-trimmed edge without creating lots of
small wrinkles. Once again, these have to be mashed down to
make them as unnoticeable as possible. The resulting wrinkle
lines should be small enough to be invisible from a distance
of one or two feet.
On the right stabilizer half, we'll iron
down the edges with stretching, and trim the excess afterward.
To get all the wrinkles out at the tip we'll need to use some
pretty aggressive stretching, and preferably use the trim iron
because of its higher heat and ease of maneuverability. Also,
unless you're blessed with three hands, you'll probably need
to grip the stabilizer between your knees while you're working
on it. Seal the edges down slightly beyond the half thickness
point, and then trim off the excess at the 50% point using the
tip of a #11 blade.
If you cut into the balsa slightly, don't worry about it.
When we cover the top left half of our
stabilizer, we'll use exactly the same procedure we used on
the bottom. Just be sure to gauge the pre-trim to overlap the
bottom covering edge so there is no exposed wood at the seam.
Finally, we'll cover the top right, once
again with the same strict adherence to the stretch rule that
we used on the bottom. We'll need to use even more stretching
at the tip because we want to seal down at least 75% of the
radius to get a good overlap of the bottom edge. Now we're
suddenly facing a nasty problem. How do we trim off the excess
of the top covering without cutting into the bottom? If we cut
through both layers, we'll create an opening for exhaust oil
to get through, and the balsa will soak it up like a wick.
This will dissolve the covering adhesive at this point, and
we'll never get it to stick again.
The answer is to use the tip of a new #11 blade. In order to cut through only the top of two layers of
covering, you must use extremely
light cutting pressure. The only thing I've found that will
work with this little pressure is a new blade. This is
something you need to practice if you haven't done it before.
You'll be amazed at how light a touch will produce a
successful trim. Before long, however, the blade will stop
cutting at this pressure level. At this point, do not
increase the pressure, because you'll end up cutting both
layers; instead, switch to another new blade. Remember, the
"used" blade is still equivalent to a new blade for
all other building applications.
I stated earlier that I may use 30 or
more #11 blades to cover a model. For those readers who were
perplexed by this, the reason should now be obvious. It is a
constraint imposed by strict application of the stretch rule
to all phases of
covering, even the seams and edges. Another way of stating
this is "zero tolerance for wrinkles."
The last sentence in the first paragraph
of this section should now be clear. How extensively you apply
the stretch rule is a matter of personal choice. Comparing the
left and right sides of a "hypothetical" stabilizer
was just a device to make you think about this choice. I
believe it makes sense to follow the rule as much a possible
when covering larger surfaces of a model using the main
sealing iron. You'll end up saving time if you don't have to
fight the wrinkles. In the case of the edges and seams, I'm
hesitant to get into value judgements. If you enjoy
getting all the wrinkles out, as I do, then the stretch rule
techniques I described to cover the right stabilizer half (and
lots of Xacto blades) are appropriate. If, however, you prefer
the pre-trimming methods, and recognize that tiny wrinkle
lines will seem inconsequential after a few landing scrapes,
then there is no reason to change to a different technique.
THE STRETCH RULE
In the preceding section about applying
the Stretch Rule to covering with film, I used as an example a
horizontal stabilizer from a typical 40-size trainer. One half
of the stabilizer was covered with total disregard for the
Stretch Rule, while the other half was covered using diligent
application of stretching at every
phase of the operation. This was simply a contrived example to
show extremes, with the hope of convincing you that the side
covered with stretching would come out looking a whole
Now the tail feathers of an airplane are
of a size and shape that make the stretching technique fairly
easy to apply. What about wings and fuselages, and how do the
size and shape of the covering piece affect the stretching
It should be obvious that a long, narrow
strip of covering will be much easier to stretch uniformly
than a large square. For this reason, a fuselage is probably
the easiest place to use the Stretch Rule. While covering the
side of a fuselage, for example, you can simply tack the
covering down at the nose and then pull firmly at the tail
while you iron it down, working from front to rear and keeping
it under strong tension the whole time.
Wings, however, are another matter. One
approach is to get a large group of family members together or
invite your flying buddies over for beer and pizza. With a
large crew of helpers standing around the perimeter of the
wing tugging on the covering, you can simply iron it down
while holding a beer in your free hand.
On those occasions where the team
approach is not possible, the solitary modeler has to do
things a bit differently. The most effective technique I know
is to "build in" the stretch. Once again, let's
illustrate this by example, using the wing from our
hypothetical 40-size trainer. The wingspan is about 60",
and there is a dihedral joint in the middle so we'll have to
cover the left and right panels separately.
Begin by cutting your piece of covering
so there is at least 1" excess along the leading and
trailing edges and wingtip. The excess will provide gripping
points to build in the stretch. Align the covering carefully
at the wing center (Hint: a strip of Scotch Safe Release
masking tape along the dihedral joint makes an excellent
positioning guide). Iron down about a 1" diameter spot at
the trailing edge center joint. Pull the covering very
strongly at the leading edge center joint and tack it down
here too. Now you can iron down a 1" strip along the full
center joint. Next we need to attach the covering at the
wingtip while stretching it strongly spanwise (Another hint:
even if the plans don't call for it, installing sheeting
between the outer two ribs can make a wing much easier to
cover). You can stretch/tack the corners or the center first,
whichever seems to work best. Just be sure you're stretching
along the span at every point that you iron down.
We've now "built in" a spanwise
stretch, next we have to do the same thing chordwise. Stretch
the covering at the leading edge at the middle of the wing
panel and tack it down just behind the rounded contour. Go to
the point directly opposite at the trailing edge, stretch and
tack. Now repeat this procedure for the leading and trailing
edges at the two quarter span points. Finally, split the
distances again the tack down the four remaining one-eighth
span points. You've now built in enough stretch that you can
iron the covering to all the wing sheeting without creating
wrinkles. You'll see wrinkles in the open wing bays at this
point, but these will disappear miraculously when you shrink
the bays with the heat gun.
In the previous article I defined the
Stretch Rule as a "rule" so that it can be applied
to every operation in covering, including edges and seams.
When the rule is followed at a fanatical level, it precludes
the pre-trimming of edges and seams before they are ironed
down. Obviously, if an edge is already trimmed, it can't be
stretched because the excess covering used to do the
stretching is gone. The consequence of this is that trimming
must be done on covering that is already sealed. At least half
the time, this means cutting covering that has another layer
of covering under it. I pointed out that trimming the top
layer and not cutting the bottom layer is a technique that has
to be learned through practice, and requires lots
of new #11 blades.
A fortunate property of rules is that
they always have exceptions. I'd like to temper the discussion
about edge and seam trimming by pointing out that there are
numerous instances where pre-trimming is not only acceptable
but even preferable. An appropriate example can be found in
the trainer wing we just talked about. Since the leading and
trailing edges are perfectly straight, you can pre-trim them
and iron down the edges without fear of wrinkles. How you
handle the wingtip depends on its shape. If it's curved,
you'll need to leave the excess attached and use the Stretch
Rule method if you hope to avoid wrinkles. There have been
numerous occasions where I've combined the techniques and used
pre-trimming on the leading and trailing edges and stretching
on the wingtips.
One of the best opportunities I can think
of for pre-trimming is the fuselage of an Ugly Stik. This is
probably the easiest fuselage imaginable to cover.
(Ironically, the triangular wingtips of an Ugly Stik present
one of the more difficult covering challenges.) The bottom is
perfectly flat from nose to tail, and the top sections are
straight in front of and behind the wing saddle. The saddle
itself is the only curvature present. With a little careful
measurement and planning, you could pre-cut all
the covering pieces to their exact shapes for this fuselage,
and have no trimming to do whatsoever.
The bottom line here is that a rule is a
rule in name only. You have to use your best judgement when to
apply it and when not.
USING THE TRIM IRON
The Top Flite Trim Seal Tool (its
official name) described in Part 2 costs a bit less than 20
bucks. It consists of a 6" cylindrical plastic handle
with an electrical cord at the back end and a 3" metal
shank on the business end. You are provided with two different
sealing tips, either of which can be attached to the shank
with a set screw. The handle has a two-pole switch to select
high or low heat settings.
One of the sealing tips is rounded, and
is designed for sealing concave surfaces such as fillets. The
other tip is flat, shaped much like the sole of a shoe. As
long as I’ve been using this tool I have yet to need the
rounded tip. For all the applications discussed here, use of
the flat tip should be assumed.
No information is provided about the
actual temperatures of the high and low settings. All I know
is the high setting is hot!
If used carelessly it will easily burn a hole right through
your covering. It will burn through your skin even faster than
that. I use the high setting about 99% of the time, so assume
the high switch position for everything discussed here.
One more comment about the trim iron
itself. The hot metal shank is insulated from the plastic
handle by a bored out wooden dowel. The tool carries a
two-year warranty, and very shortly after the warranty expires
the wooden insulator will crack, allowing the shank to wobble
uselessly. The solution is to tighten a small metal hose clamp
around the wood piece to provide several more years of
My fondness for the trim iron comes from
the many uses I've found for it that go beyond the
manufacturer's intentions. Top Flite positions the purpose of
this tool to apply pin stripes and graphics (which I almost
never use it for) and to cover "nooks and crannies"
where the large iron won't fit. Examples might be inside
cowls, cockpits, or the fuselage area between the fin and
stab. The advantage of a smaller size iron for these
situations is obvious.
In my opinion, the most important
usefulness of the trim iron, by far, lies in covering compound
curves. The principal examples are wing and tail feather tips.
It is the extremely high heat output of the tool in its high
switch position that makes this possible. Remember, I said the
tip is hot enough to damage the covering. The important thing
to realize is that when the heat is very high, contact time
must be low. The technique I use when heating covering on a
compound curve is a "patting" or "tapping"
motion in which the hot tip is applied and removed repeatedly.
This is combined with a very strong stretching of the heated
area. The constant on-off application of this high heat
transforms the film to a rubber-like consistency (without
melting it) that can be stretched over any imaginable surface.
Let's consider an example. Assume you're
covering a wing with a symmetrical airfoil and a shaped,
rounded wingtip much like the one on a P-51. You started by
covering the wing bottom using the stretching technique
described in the preceding section. The covering is smoothly
sealed to the entire airfoil, but as you begin ironing close
to the rounded wingtip you begin to see serious wrinkles
developing. The wrinkles will be most prominent at the wingtip
leading edge where the curvature is greatest. At this point,
stop! Put your main sealing iron down, you won't need it again
until you cover the top of the wing. From this point on we
will use only the trim iron.
I'll assume you're right handed (if not,
reverse everything). Grab hold of the excess covering near the
wingtip leading edge with your left thumb and forefinger, and
stretch the covering strongly using your other three fingers
as a lever point. (Hint:
use the flat portion of your fingers, not the knuckles; you're
going to stretch so hard that knuckles will dent the balsa.)
As you stretch the covering, use your right hand to tap at the
developing wrinkle with the hot trim iron. The wrinkle will
vanish, but hold the stretch for a few seconds after removing
the heat while the film cools. The first wrinkle will be gone,
but it will be replaced by two or more new ones further along
the curvature. Repeat this same procedure on the new wrinkles.
Every time you remove a wrinkle, you'll create more new ones
than you fixed. If this sounds tedious, it is. It's a very
slow back and forth process in which you work around the
perimeter of the wingtip numerous times, making no more than
about 1/8" progress per pass. Do not, under any
circumstances, make axial cuts in the excess covering to
relieve stress during this process.
Eventually, you'll have de-wrinkled the
covering all the way around to the centerline of the wingtip,
at which point you can trim off the excess film. The covering
on the wingtip will be smooth and wrinkle free, and the excess
you trimmed off will consist of nothing but wrinkles. It gets even more difficult when you cover the top of
the wing. This time you have to stretch a little beyond the
wingtip centerline in order to overlap the bottom covering.
Also, when you trim, you're cutting covering over covering,
and a new #11 blade
Covering the tips of tail feathers is the
same process, but it's easier because the total curvature is
much smaller. The process I described here can be used
anywhere there is a compound curve. Fuselages that are tapered
and rounded in the nose area provide another good opportunity
to use this technique.
I'll briefly mention a few more
"unadvertised" uses for the trim iron. Whenever I
create an overlap seam using the main iron, I always finish by
making a quick pass over the seam with the trim iron. The
extra heat seals the seam more securely so it will never come
apart by itself.
The trim iron is much more effective than
the main iron in sealing covering to sharp edges. Examples are
the ends of ailerons, elevators, and rudders, hatch openings,
cowl and cockpit edges, etc.
It's also a wonderful tool for repairing
dents and scratches. If you ever tried to iron out a hangar
rash ding using the main iron, you may have been disappointed
in the result. A few taps with the trim iron at high heat will
usually make the flaw disappear completely. For more severe
dents, poke a tiny hole in the covering, rub a little water on
the dent so the underlying wood gets moistened, and then apply
the hot trim iron.