Jerry
Still on the filling but I have thought the article excellent as well, ofte
n turning back to re-read parts.
but the hardest part at first was :-
=22The right way to do it is to fill all of the surface (meaning like one sid
e 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 unif
orm plain ONCE, without the need of refilling again.=22
Strimmer lines as advocated by Graham Singleton help. I used them and a 2=22
pipe as a roller with thin plastic sheet between filler and roller as advoc
ated by Kingsley Hurst.
JR (Bob) Gowing UK Kit 327 in Oz
----- Original Message ----- )
From: Jerry Rehn
To: europa-list@matronics.com
Sent: Friday, September 19, 2008 4:06 AM
Subject: Europa-List: Finishing tips
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 thei
r 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 t
o be written by people who are writers rather then builders and never actua
lly 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 fi
lled 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 wi
th 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 o
n an award winning Lancair IV P for Mike Shooner under the command of Arthu
r Gallant who himself received Oshkosh Grand Champion for his Lancair 320.
I learned a lot on that project. The following describes in great details t
he 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 sur
face, second, we perfect the surface for the paint application. In other ar
ticles you probably read directions about looking for =22The Low Spots=22 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 i
t. 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 i
t is to fill all of the surface (meaning like one side of a wing or a fusel
age) 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, withou
t the need of refilling again. The amount of filler to sand off may seem ov
erwhelming 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 thr
ough the filler. Some airplanes, because of their building process will hav
e deep depressions, like in the place of spars or joints, which would be di
fficult 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 b
e 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 crea
te hard spots and uneven sanding properties. The picture below shows a good
example of this first head start fill where we had a significant depressio
n 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 st
art filling, Even thou this step is not absolutely necessary it is very hel
pful in guiding where to direct more sanding and when to change to finer sa
ndpaper grade. The filler becomes translucent as it gets thinner and the bl
ack color starts showing through as dark spots indicating you are getting c
lose to the skin surface.
The filler is a plain mix of epoxy and glass micro balloons, commonly cal
led =22micro=22. It's not all that easy to work with but it's the lightest fill
er there is with good strength and stability after its cured. I would not s
ay you can not use one of the ready to apply premixed epoxy fillers like Su
perFil but be aware that despite all the advertising hype this filler is ab
out twice as heavy as home made micro and it will cost a lot more as a numb
er 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 a
bout as much as I can handle. Mixing is done with a classic flat paint mixi
ng stick and in a slow motion, trying to mix it fast will not get the job d
one 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 beco
mes 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 micro
baloons is that the surface turns glossy when you let it sit a minute that'
s the extra epoxy migrating to the surface. Properly mixed micro spreads wi
th moderate difficulty and fairly high amount of pressure is required. If i
t spreads easily it's too wet.
Be aware that micro has one distinct characteristic, that is, the two mat
erials - microbaloons and epoxy tend to separate from one another when stil
l. This characteristic is good for the fact that the resin migrates onto th
e 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 prev
iously 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 pe
ople squeegee pure resin on the surface to wet it out just prior applying t
he 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 th
e micro (the mixed batch) to the middle of the surface and then trowel it i
n the direction toward yourself first to one edge, then run around and to t
he 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 b
y the top part of the blade with all fingers on top and thumbs in the botto
m 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 i
s what pushes the air bubbles that got mixed in to the micro out of the mic
ro. You will notice that at first it doesn't seem to want to stick and want
s 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 lo
ng 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 e
asily, remember this important tip, if you make a pass and you see the micr
o breaking up make a pass in the opposite direction, that will smooth it ri
ght up, but then just stop fussing with it. The thickness if the coat shoul
d 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 th
en 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 ho
w thick it is , just dip a Popsicle stick in to it. As a general rule, star
t the filling on smaller, single curve surfaces like the tail, then large s
ingle curve surfaces like wings, and last the most difficult compound curve
d parts on the fuselage, engine cowl, wing fillet so that the level of diff
iculty goes up along with your filling and sanding learning curve. On filli
ng the compound shapes the flat metal trowel will not work, for those you n
eed to use a flexible plastic squeegee. With 4 fingers on top and the thumb
against the bottom you can =22cap=22 the squeegee so it follows the curve. Hom
e 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 onc
e, try to divide it so bring the edges to a break like a corner where blend
ing of one fill into another is easiest. If the fuselage is oval, do the br
eak 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 s
hort as needed, Many builders don't realize (or disregard) the importance o
f the right sanding tools. Trying to sand with a chunk of triple folded san
dpaper 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 spendin
g hundreds of hours on finish sanding his airplane and yet you won't be abl
e to find a decent sanding board in his shop. Make yourself two basic sandi
ng 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 mat
erial. You need to attach an 1 =BC x 1 =BC aluminum square tube or a =22C=22
channe
l to the back side of it with some screws, that will keep it straight and a
lso 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 anywhe
re but it will make you work a lot easier when the sanding board bends unde
r the pressure and follows the curve of the surface. You will need two boar
ds, one should flex rather easy for highly curved surfaces and one should b
e fairly stiff for mild curves. The 16 inches length of these flexible boar
ds should be enough . Use thin plywood, Plexiglas, or whatever works for yo
u. 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 tim
es) 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 ed
ge 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 th
e 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 passe
s move yourself to the trailing edge side and make two passes from there, a
gain starting the first pass at 90 degrees to the previous one, and then ke
ep repeating this cycle. It is this sanding pattern, that will level out th
e 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 an
d directing more attention to them. The best way to check the trueness of t
he surface is to feel it by sliding your hand over the surface in long swee
ps in all different directions. If you have never tried this you will be am
azed to find out how sensitive your hand is to even minute surface waves an
d unevenness. Direct more sanding to the identified high areas and pass qui
cker over the low ones to speed up the process. If everything goes right, y
ou will have good straight surface just before you remove all the filler an
d 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 acros
s the whole surface. At this point you need to change the sandpaper grade t
o 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. L
uckily I have some good ones to show you what it should look like at this p
oint. On the picture below the right wing has just been filled and is curin
g 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's hard to take a picture of an all white surface, it tends to get overex
posed 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 d
one and when I switch to finer grade paper. You can see the cross hatch san
ding 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 t
wo-seater, up to about 8 hours on a big four-seater.
Compound curved surfaces are a little harder to do. Keep basically the sa
me crisscross 45 degree pattern relative to the direction of the milder cur
ve with your flexible sanding board, but pay a lot more attention to =22feeli
ng=22 the surface and sanding off the highs. By the time you do all your flyi
ng surfaces you will be pretty good at it.
Detailing
There are three kinds of details we have on the plane to deal with, fille
ts 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, tha
t way the filler is brought up to and blended to the level required to clos
e the gaps at the nose area of that control surface.
Filets are fairly easy, shape a corner of a plastic squeegee to the desir
ed 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 firs
t 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 th
e tube sanding tool that closely matches the radius to sand it smooth.
Inspection panels . In order to have good looking access and inspection p
anels, 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. F
ist make sure the covers are exact size and shape, round off the corners, s
quare corners don't look right. Apply clear package sealing tape to the bac
kside and wherever needed as a release and mount the covers into exact posi
tion. 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 th
e cover tight against the edge to create a nice line around it. Cutting dow
n the width of the mixing stick will help it to go around the corners witho
ut widening the gap and dipping the mixing stick in lacquer thinner will pr
event 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 thic
kness gaps that don't change a bit, even when you move the surface =3F Here i
s how to make them. Apply several plies of duck tape on the underside of th
e 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 wit
h 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 thro
ugh 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 r
eference so you don't lose it and sand into it later. before filling the re
st of the surface, cover the finished gap part with the duck tape to preser
ve 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 th
e 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, fi
ll 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 l
id 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 san
ding over those spots to remove the leftover fuzz. To get the right perspec
tive 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 fille
r. 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 o
f 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 th
ey will disappear, but they will not be filled, they will be bridged. Bridg
ed pinholes can cause the painted surface to develop little pimple like bum
ps 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 s
queegee pure epoxy resin over the surface. The coat is very thin and the am
ount 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 lo
w surface tension so it flows into all those small voids and because unlike
primers it doesn't contain any volatiles it doesn't shrink as it cures so
the fill is complete. The second benefit of this step is that the resins ha
rdens 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 v
ery thin coat, especially in humid condition. If yours is one of those or y
ou 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 gl
oss, and you are ready for the primer.
The primer used in this step is a =22high build up=22 kind, intended for fina
l wet sanding. There are many on the market so I can't say you must use thi
s , or cannot use that, except maybe for one. You may have seen reference i
n other articles (especially the older ones) to use =22Feather fill=22. I have
used it and I don't understand how anybody can recommend it . This is one o
f the worst products I have seen in my life.
I have been using two kinds of primers so far.
One is =22PPG K200=22 this is acrylic urethane (two part 4:1), very fast dryi
ng, easy to spray, dark yellow in color. Available from auto paint supply s
tores. It wet sands very easy, however it has rather soft properties even w
hen cured. Mishandling of parts can dent and scratch the surface rather eas
ily.
The other kind is epoxy based =22US Paint D9002 base /D3002 converter=22 (Two
part 1:1). Also dark yellow, available from marine supply stores ( www.usp
aint.com ) This primer takes longer to dry and it's a bit harder to sand bu
t 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 =22Poly-fiber Smooth Prime=22 but I plan on it
.
Wet sanding is done by still keeping the same 45 degrees crisscross patte
rn 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 =22super 77 spray adhesive=22 and spray just very light mist on it to
attach it, it will peel off without the need of heating it up. Start with 1
80 grade for fast progress but be aware 180 leaves pretty deep scratches th
at would show in the paint so switch to 220 grade for the second half of th
e job. On the contrary to some peoples believe, 1 sheet of wet sandpaper do
es not last forever, it just isn't as obvious its getting dull by looking a
t it so change the paper often.
To check the progression and quality of the surface use the squeegee tech
nique. Pour some water over the surface and squeegee it off with a rubber w
indow squeegee (Home depot has those), all the remaining pits will become i
nstantly visible. Keep sanding and checking until they are all gone. Don't
go crazy if you have a few deeper nicks left here and there, and don't sand
flat spots or dips into your surface in order to get rid of them. They wil
l be filled instead.
For this last filling use polyester based =22glazing putty=22 (auto paint sup
plies) This is very smooth creamy filler designed for minor imperfections.
Before you start filling go over the whole surface carefully and mark all i
mperfections to be filled with a pencil so you don't 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. S
queegee it on with a small metal spatula and scrape off any excess. Wet san
d 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 th
e paint to insure good paint adhesion.
This is where I leave it, I'm 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 advic
e of others.
You can distribute and link to this page freely, comments and questions a
re welcomed.
Back to main page
---------------------------------------------------------------------------
---
Checked by AVG.
08 7:06 PM
--
|