Hints and tips - filling
Having taken very great care with my tailplanes last year to put on a nice
smooth layer of filler with just my squeegee - and wished (after about 2 hours
of sanding it all down again) that there was an easier way of controlling the
thickness of the filler you were putting on, I tried the 'fishing line
technique' for the first time on one of my flaps and an aileron last week. It
was so successful that it is worth repeating again as well as mentioning a few
things that might prompt a bit of thought on the subject.
Firstly though, an interesting observation.
The instructions recommend that you bake all your surfaces before you fill. This
seemed a bit strange to me, as with my limited knowledge of composite
construction I had always believed that the filler would achieve a better
'molecular bond' to the 'not yet baked' skins to those that were baked first
then filled.
Another builder also thought this and contacted SP systems (manufacturers of the
epoxy). They seemed to also think that once cooked you would lose some of the
bonding properties of the epoxy. They also said that once any trapped moisture
has been baked out moisture does not ingress back in.
Question is does it matter either way?
I am baking after filling - by leaving the surfaces in my garage for a few weeks
during the summer months - last year it was a constant 40-45 deg C. Maybe as the
temperatures and humidities do not get so high/low (respectively) in the U.K and
you perhaps can only afford to bake for 24 - 48 hours in your own homemade
ovens, it is not long enough to ensure moisture evaporation from both skin and
filler layers together.
It would be interesting to get other opinions/experiences.
Back to the filling. The fishing line technique works! I used 60 lb breaking
strain (.75 mm). For those not familiar, you weigh each end of a couple of
lengths of line - each long enough to drap over the surface being filled, then
with the lines spaced apart 6 to 8 in. you slap on your filler - working it well
onto the surface (I abraded and cleaned the surfaces prior to filling as per
instructions). Then with a steel blade - I used a drywall finishing blade, you
run the blade down the 'tram lines'. Voila, the filler down between the lines is
.75mm thick. Move one line over the other another 6 - 8 in. and repeat the
process.
Very quickly you will have the surface complete although you will have to just
'close up' where the lines were by just running your sqeegee along them.
The wings should be more interesting. There are areas where there are varying
heights of skin, ie. the root area and the wingtips. I am planning on putting on
filler in stages building up the low areas first in steps of .75mm until the
final 'top coat' which should be pretty close to the final profile by then -
thus minimising sanding.
Perhaps those of you who have filled their wings are somewhat bemused by the
last paragraph - I don't think it will be as simple as perhaps I make it sound!
Still that's the plan.
A quick word about filler consistancy. There was much debate about the
wetness/dryness of filler on the forum a couple of months ago. I found that -
like the instructions said, a double whipped cream consistancy is about right -
perhaps slightly dryer. I found that if you can still stir the mix with your
'tongue depressor' mixing stick then it is probably too wet - and your thumb and
forefinger ache! I used a 1/2 in. dowel as you can get a firm grip. When it
sticks to the sides of the pot and leaves a hole in the middle thats about
right. Also, if after around 10 seconds without stirring it is just beginning to
'semi-gloss' over, that's also a good visual indication.
Too dry it will roll up behind the blade - and be too soft perhaps for a good
hard skin, too wet and it will be heavy and take forever to sand.
These are just a few thoughts and experiences I had which I thought I might
share. I would welcome any further comments.
Happy filling!
Martin Tuck
#152
Wichita, Kansas
U.S.A
|