I found this to be a very good article.
Jerry
_____
Finishing a composite airplane
Foreword
I wrote this because I see a lot of people struggling with finishing
their
airplanes and I don't see anything well written on the subject anywhere.
I
have seen a number of articles in magazines and books but they all seem
to
be written by people who are writers rather then builders and never
actually
did the job, let alone would be good at it. The latest example of such
article is in Sport Aviation 2/2003 named The Joy of Sanding, four pages
filled but nothing helpful about how to do the job. I, on the other
hand, am
no writer and English is a second language for me to top it off, so bear
with me. My own experience comes from building composite tools and
production parts for kit airplanes as a business and from doing all the
finish work on an award winning Lancair IV P for Mike Shooner under the
command of Arthur Gallant who himself received Oshkosh Grand Champion
for
his Lancair 320. I learned a lot on that project. The following
describes in
great details the techniques I use to obtain a perfect straight wave
free
airfoil shape on wings and the fuselage, and do it with the minimum
effort
and cost without spending years on it (or so I think).
Ok, lets start filling.
The process goes basically in two stages, first, we shape/contour the
surface, second, we perfect the surface for the paint application. In
other
articles you probably read directions about looking for "The Low Spots"
with
all kinds of tricks on how to identify them and fill them, then sand,
then
recheck and refill again and again and again. This is the wrong way to
do
it. Not only is this technique time consuming, achieving a wave free
surface
by a patchwork of fillings is just about impossible. The right way to do
it
is to fill all of the surface (meaning like one side of a wing or a
fuselage) at once with a coat of filler sufficiently thick enough to
fully
fill all the lows, then sand the whole surface as one uniform plain
ONCE,
without the need of refilling again. The amount of filler to sand off
may
seem overwhelming at first but the use of proper sanding technique
described
later on makes this task rather effortless. Using this technique you
never
look for low spots, instead you keep sanding until the highs start
coming up
through the filler. Some airplanes, because of their building process
will
have deep depressions, like in the place of spars or joints, which would
be
difficult to fill sufficiently deep and yet smooth in one fill. The
builders
natural tendency would be just to do the first big fill, sand to the
shape
and then refill the areas of these low depressions as needed. It needs
to be
done the other way around, you need to fill these deep depression first,
however that first fill must still stay below the intended finished
surface
height, sanding through the top fill into the first filler coat would
create
hard spots and uneven sanding properties. The picture below shows a good
example of this first head start fill where we had a significant
depression
in the skin in the spar area and the joggle joint on the leading edge.
I strongly recommend spraying a light coat of black primer, before you
start
filling, Even thou this step is not absolutely necessary it is very
helpful
in guiding where to direct more sanding and when to change to finer
sandpaper grade. The filler becomes translucent as it gets thinner and
the
black color starts showing through as dark spots indicating you are
getting
close to the skin surface.
The filler is a plain mix of epoxy and glass micro balloons, commonly
called
"micro". It's not all that easy to work with but it's the lightest
filler
there is with good strength and stability after its cured. I would not
say
you can not use one of the ready to apply premixed epoxy fillers like
SuperFil but be aware that despite all the advertising hype this filler
is
about twice as heavy as home made micro and it will cost a lot more as a
number of gallons of filler are used. The epoxy resin used to mix micro
must
be one of the low viscosity types that will allow high ratio of
microbaloons
to be mixed in. I have used Aeropoxy and Jeffco which both work well. I
mix
it in a large salad bowl about 2/3 to 3/4 of a gallon at a time which is
about as much as I can handle. Mixing is done with a classic flat paint
mixing stick and in a slow motion, trying to mix it fast will not get
the
job done any sooner, you will only blow half of the microbaloons in to
the
air. The finished mix must be fairly dry, keep adding microbaloons until
it
becomes difficult to mix. As long as the mix is visibly flowing and
leveling
by itself its still too wet, another indicator the mix still needs more
microbaloons is that the surface turns glossy when you let it sit a
minute
that=92s the extra epoxy migrating to the surface. Properly mixed micro
spreads with moderate difficulty and fairly high amount of pressure is
required. If it spreads easily it's too wet.
Be aware that micro has one distinct characteristic, that is, the two
materials - microbaloons and epoxy tend to separate from one another
when
still. This characteristic is good for the fact that the resin migrates
onto
the surface being filled creating a real good bond to it. The bad part
is if
you have to refill an area, the resin migrates into the surface of the
previously applied micro creating hard spots when sanding it. These hard
spots create raised areas, a real pain to deal with so try to avoid
that.
Some people squeegee pure resin on the surface to wet it out just prior
applying the micro. I see no reason or purpose of that, I think they got
it
---From the way the plaster skim coat is applied on the drywall.
To apply the micro to the surface, use a 6-inch metal trowel. Blob all
the
micro (the mixed batch) to the middle of the surface and then trowel it
in
the direction toward yourself first to one edge, then run around and to
the
other side edge. Always go in the direction of the curve. The trowel is
run
very flat to the surface and good pressure, you hold it by both hands by
the
top part of the blade with all fingers on top and thumbs in the bottom
the
get a good grip, bend the handle up about 30 degrees to get it out of
the
way. The low angle and plenty of pressure are important because that is
what
pushes the air bubbles that got mixed in to the micro out of the micro.
You
will notice that at first it doesn't seem to want to stick and wants to
roll
off, but then when the surface starts to get wetted out, it goes on
pretty
smooth. You don't have much time thou, if you fuss with it too long or
try
to redo an area you did a few minutes ago, it will start to tear up,
lift
off and break up. This is because o lot of resin migrated onto the
surface
and the micro starts to slide on it and unstuck from the surface easily,
remember this important tip, if you make a pass and you see the micro
breaking up make a pass in the opposite direction, that will smooth it
right
up, but then just stop fussing with it. The thickness if the coat should
be
somewhere between 1/8 to =BC of an inch, you shouldn't have any lows on
your
surface deeper then 1/8 of an inch .A little bit too much is better then
a
little bit not enough because it's still easier to sand off some extra
then
having to refill again. It takes some practice, if you want to see how
thick
it is , just dip a Popsicle stick in to it. As a general rule, start the
filling on smaller, single curve surfaces like the tail, then large
single
curve surfaces like wings, and last the most difficult compound curved
parts
on the fuselage, engine cowl, wing fillet so that the level of
difficulty
goes up along with your filling and sanding learning curve. On filling
the
compound shapes the flat metal trowel will not work, for those you need
to
use a flexible plastic squeegee. With 4 fingers on top and the thumb
against
the bottom you can "cap" the squeegee so it follows the curve. Home
depot
sells nice ones in a tri pack of 5, 4 and 2.5 inch wide. On a part like
a
fuselage where you will not be able to fill the whole surface at once,
try
to divide it so bring the edges to a break like a corner where blending
of
one fill into another is easiest. If the fuselage is oval, do the break
where the curve is the sharpest, its much easier to blend it there then
on
the shallow curve. It should take about 2 hours to fill one wing panel
on a
small two-seater, up to about 4 hours on a big four-seater.
Sanding
There are three aspects of the sanding technique that will enable you to
arrive to that perfect straight wave free surface.
First, you must use the right size an shape sanding tool for a given
job,
different shapes require different tool, boards, blocks, tubes, long and
short as needed, Many builders don't realize (or disregard) the
importance
of the right sanding tools. Trying to sand with a chunk of triple folded
sandpaper pushed along with bear hand is not going to work too well. You
will find it a great paradox if you visit somebody's workshop who claims
spending hundreds of hours on finish sanding his airplane and yet you
won't
be able to find a decent sanding board in his shop. Make yourself two
basic
sanding boards 2,3/4 inch wide 16 and 32 inches long, for the length of
one
and two strips of sandpaper. I think the best is to use that white
shelving
material. You need to attach an 1 =BC x 1 =BC aluminum square tube or a
"C"
channel to the back side of it with some screws, that will keep it
straight
and also gives you a good grip so you can hold on to it. The 32 inch one
will be used on the wings, 16 inch on the tail, control surfaces and
flat
parts of the fuselage. For compound surfaces you will need to make
flexible
sanding boards. Not many people know this as I have never seen it
mentioned
anywhere but it will make you work a lot easier when the sanding board
bends
under the pressure and follows the curve of the surface. You will need
two
boards, one should flex rather easy for highly curved surfaces and one
should be fairly stiff for mild curves. The 16 inches length of these
flexible boards should be enough . Use thin plywood, Plexiglas, or
whatever
works for you. Get the real sandpaper from auto paint supply store, the
one
that comes in 16 inch long strips, they usually carry the best, the 3M
brand
is pretty sure bet. The sandpaper grades needed are 36, 80 and 100. Glue
it
on with 3M Super 77 spray adhesive (Home depot), spray on only a light
coat
of it, that stuff sticks like hell. To remove the worn out paper, heat
it up
with a heat gun, it will peel right off.
On tube shape sanding tools the strips will not work, for those, get the
big
sheets for floor sanding from Home depot.
The second aspects of the sanding technique is the proper sanding
pattern.
For an airfoil shape surface, hold the sanding board parallel (at all
times)
to the span and sand at 45 degrees angle, use the longest (reasonable)
stroke, across the whole cord if possible. Start at the root end leading
edge side and move slowly to the tip end with each stroke moved about an
inch or two, creating a sort of a tall tight zigzag pattern. When you
get to
the tip, go back to the root and start another pass, this time sanding
at 45
degrees the other way - 90 degrees to the first pass. After these two
passes
move yourself to the trailing edge side and make two passes from there,
again starting the first pass at 90 degrees to the previous one, and
then
keep repeating this cycle. It is this sanding pattern, that will level
out
the highs and lows into a nice even flowing plain and create the desired
wave free surface.
The third aspect is you checking the surface frequently for high areas
and
directing more attention to them. The best way to check the trueness of
the
surface is to feel it by sliding your hand over the surface in long
sweeps
in all different directions. If you have never tried this you will be
amazed
to find out how sensitive your hand is to even minute surface waves and
unevenness. Direct more sanding to the identified high areas and pass
quicker over the low ones to speed up the process. If everything goes
right,
you will have good straight surface just before you remove all the
filler
and the high spots of the skin start showing through the filler. Keep
sanding off the excess filler until the highs start to show up as dark
spots
across the whole surface. At this point you need to change the sandpaper
grade to 80 for the final sanding stage so you can remove the deep
scratches
left by the 36 grade. Do the final once-over with a 100 grade and that's
as
fine as you need to go on this. They say a picture is worth a thousand
words. Luckily I have some good ones to show you what it should look
like at
this point. On the picture below the right wing has just been filled and
is
curing while I'm sanding the left one
This is a close up of the filed right wing
Here the left wing is sanded and the right wing is just getting started.
It=92s hard to take a picture of an all white surface, it tends to get
overexposed and the details are lost but looking at the pattern of the
dark
spots will give you pretty good idea what it should look like.
This is the horizontal stabilizer just at the point when the shaping is
done
and when I switch to finer grade paper. You can see the cross hatch
sanding
pattern there. The dark spots will get quite a bit darker by the time
all
deep scratches are gone.
It should take about 4 hours to sand one wing panel to shape on a small
two-seater, up to about 8 hours on a big four-seater.
Compound curved surfaces are a little harder to do. Keep basically the
same
crisscross 45 degree pattern relative to the direction of the milder
curve
with your flexible sanding board, but pay a lot more attention to
"feeling"
the surface and sanding off the highs. By the time you do all your
flying
surfaces you will be pretty good at it.
Detailing
There are three kinds of details we have on the plane to deal with,
fillets
where components come together and meet at different angles, fitting of
access and inspection panels and uniform control surfaces gaps.
Take a note here, if you decide to do the control surfaces gaps, you
need to
do them Before the general surface filling of the control surfaces, that
way
the filler is brought up to and blended to the level required to close
the
gaps at the nose area of that control surface.
Filets are fairly easy, shape a corner of a plastic squeegee to the
desired
radius and use that to shape the applied micro into the fillet radius .
Run
the squeegee back and forth in alternating directions very flat at first
to
evenly distribute the micro, then start standing it up with each pass to
gradually remove the excess micro and arrive to the final radius. Use
the
tube sanding tool that closely matches the radius to sand it smooth.
Inspection panels . In order to have good looking access and inspection
panels, or any other openings like gear doors and baggage compartments
doors
or even an engine cowling if its fitted flush with the fuselage, you
need to
create a small even gap around them. Here is the trick how to do that.
Fist
make sure the covers are exact size and shape, round off the corners,
square
corners don=92t look right. Apply clear package sealing tape to the
backside
and wherever needed as a release and mount the covers into exact
position.
Now squeegee the micro all around the recess, or over the whole cover if
the
height of it is not even, and then run a mixing stick all around the
cover
tight against the edge to create a nice line around it. Cutting down the
width of the mixing stick will help it to go around the corners without
widening the gap and dipping the mixing stick in lacquer thinner will
prevent micro from sticking to the stick and help to keep the line edges
nice and smooth.
Control surfaces gaps Ever heard about those incredible even Quarter
thickness gaps that don=92t change a bit, even when you move the surface
?
Here is how to make them. Apply several plies of duck tape on the
underside
of the trailing edge to build up the desired gap thickness. Put clear
packaging tape over it (its slicker and slides better), you will
probably be
able to handle only one side at the time. Apply plenty of micro to the
nose
of the control surface to fill the intended gap and then some. Cover the
micro with a plastic sheet, using thicker stiffer kind on this works
better,
install the control surface into its hinges and slowly move it up and
down
through the whole travel so that the excess micro is pushed out. Viola,
after the duck tape build up is removed, you have a beautiful small and
even
gap through the whole control surface travel. Before you start removing
the
excess micro that got pushed off mark the ends of the travel with a good
line as reference so you don=92t lose it and sand into it later. before
filling the rest of the surface, cover the finished gap part with the
duck
tape to preserve it.
This concludes the first stage, the surface shaping.
Surface finishing
is the second stage of the work when we remove all the nicks, pits,
tears
and finally the scratches. First, vacuum the surface, you need to get
all
the sanding dust out of the nicks so you can see them and fill them.
Micro
does not work on small nicks, it has too coarse consistency. You can use
the
Superfil which is much smoother or you can mix your own filler by adding
cab-o-sil along with microbaloons. To mix this filler, get a coffee can,
fill it 1/3 with microbaloons first, then 1/3 Cab-o-sil. Leave the last
third empty so you can mix it well and add more of either substance if
you
want to adjust the ratio, throw some wood blocks in to help mixing it,
put
the lid on and mix it well. Fill the nicks with a small one inch metal
spatula. Squeegee the filler on to fill the nick but scrape all the
excess
---From the surface, it would be very hard to sand. That way you will do
only
light sanding over those spots to remove the leftover fuzz. To get the
right
perspective what size of the nicks is big enough to fill and which is
too
small to be bothered with, use the rule, if you can see it from two feet
away, fill it. Another way to look at it if the nick is no deeper then a
scratch left by a 36 grid sandpaper.
Pinholes
These are tiny voids, bubbles and pits caused by air mixed into the
filler.
They are invisible until the minute you start spraying the first coat of
primer. They can give you a big headache if you attack them the wrong
way.
You might have heard horror stories of builders spraying coat after coat
of
primer trying to get rid of them. The bottom line is, you cannot fill
the
pinholes by spraying. Yes, if you keep spraying long enough, eventually
they
will disappear, but they will not be filled, they will be bridged.
Bridged
pinholes can cause the painted surface to develop little pimple like
bumps
on the sun when the paint softens some and the trapped air expands with
the
heat.
Luckily, we have a simple effective way how to deal with pinholes before
we
even see any. Vacuum the surface real well to remove any dust and then
squeegee pure epoxy resin over the surface. The coat is very thin and
the
amount of resin is mall, you are basically just wetting the surface,
give it
some time to soak in and squeegee off all the excess. The resin has very
low
surface tension so it flows into all those small voids and because
unlike
primers it doesn=92t contain any volatiles it doesn=92t shrink as it
cures so
the fill is complete. The second benefit of this step is that the resins
hardens the top shell of the micro, making it more durable.
A word of caution, many epoxy resins do not cure well and stay gummy at
very
thin coat, especially in humid condition. If yours is one of those or
you
are not sure, use the West system epoxy for this.
When this top coat is cured sand it lightly with 100 just to break the
gloss, and you are ready for the primer.
The primer used in this step is a "high build up" kind, intended for
final
wet sanding. There are many on the market so I can=92t say you must use
this ,
or cannot use that, except maybe for one. You may have seen reference in
other articles (especially the older ones) to use "Feather fill". I have
used it and I don=92t understand how anybody can recommend it . This is
one of
the worst products I have seen in my life.
I have been using two kinds of primers so far.
One is "PPG K200" this is acrylic urethane (two part 4:1), very fast
drying,
easy to spray, dark yellow in color. Available from auto paint supply
stores. It wet sands very easy, however it has rather soft properties
even
when cured. Mishandling of parts can dent and scratch the surface rather
easily.
The other kind is epoxy based "US Paint D9002 base /D3002 converter"
(Two
part 1:1). Also dark yellow, available from marine supply stores (
www.uspaint.com ) This primer takes longer to dry and it=92s a bit
harder to
sand but it is very hard and durable when fully cured. I myself prefer
the
better durability, even if it takes more work.
I have not had a chance to try "Poly-fiber Smooth Prime" but I plan on
it.
Wet sanding is done by still keeping the same 45 degrees crisscross
pattern
but the sanding blocks are a lot smaller, 11x3 inches for large parts
5.5x3
for the smaller ones. I use pieces of hard foam as sanding blocks. Use
the
same "super 77 spray adhesive" and spray just very light mist on it to
attach it, it will peel off without the need of heating it up. Start
with
180 grade for fast progress but be aware 180 leaves pretty deep
scratches
that would show in the paint so switch to 220 grade for the second half
of
the job. On the contrary to some peoples believe, 1 sheet of wet
sandpaper
does not last forever, it just isn=92t as obvious its getting dull by
looking
at it so change the paper often.
To check the progression and quality of the surface use the squeegee
technique. Pour some water over the surface and squeegee it off with a
rubber window squeegee (Home depot has those), all the remaining pits
will
become instantly visible. Keep sanding and checking until they are all
gone.
Don=92t go crazy if you have a few deeper nicks left here and there, and
don=92t
sand flat spots or dips into your surface in order to get rid of them.
They
will be filled instead.
For this last filling use polyester based "glazing putty" (auto paint
supplies) This is very smooth creamy filler designed for minor
imperfections. Before you start filling go over the whole surface
carefully
and mark all imperfections to be filled with a pencil so you don=92t
have too
look for them while your mixed putty is curing. Mix the glazing filler
in
small batches, it has very short pot life so you have only a few minutes
on
each batch. Squeegee it on with a small metal spatula and scrape off any
excess. Wet sand the spots after they cure and you are done.
There will be one more coat of primer but that is as a part of painting
.
Every paint system normally has its own primer that goes on just before
the
paint to insure good paint adhesion.
This is where I leave it, I=92m not an expert on painting so with the
paint
you are on your own.
Legal disclaimer
This is only one mans opinion and my contradict with experience and
advice
of others.
You can distribute and link to this page freely, comments and questions
are
welcomed.
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