Cheers,
I haven't seen much advice on the application of epoxy to glass fibre
(except of course a couple of US Bingelis books and chapetr 1 of the
construction book). Having completed most of the layup work (large stuff
anyhow) and being fairly content with the results....visually, I offer
the following for those behindhand:
1. I took advice on mixing epoxy from my nephew who is an SPS expert
and opted for measuring weight rather than volume because I presumed
molecular weight was the criterion for correct mixing. I used a 400gm
digital scale which I test before every weighing...a ten second delay...
and for small amounts (flox corners or microslurry of small areas) weigh
and mix in discarded Kodak film containers (4gm)...the most common being
10.0g syrup and 2.7g hardener (AEROPOXY formula) for a total of 12.7g.
Since the scale is correct to the nearest 0.1g, my greatest error is
somewhat less than 1% which is well within the limits set anywhere. This
discrepancy becomes even more insignificant in larger mixes of course. I
calculate that I have never been outside the 1% limit and am usually
under one tenth of that, thanks to a mixing chart from the computor.
2. Since I am lucky to be working in an indoor environment (Plastic
wall in a large spare bedroom) with separate air source and
temperature/humidity control, I have been able to store/mix epoxy within
20-30degC temps and below 50% hum. These I record in the log for every
mix and application.
3. I've found it vital to consult a sign which tells me what to prepare
in advance of a layup, in the way of all the needed materials and tools
BEFORE starting. One gets enured to sequential details after a while and
there's the temptation to think familiarity breeds preparation. This
leads to a desparate scurry for some aid or other when the ply is
peeling off and the pot life is approaching! The stages of prep are in
the manual.
4. I spose this is now universal, but pre-wetting layups (especially
large ones) in plastic sheeting is the answer to time constraints - that
is, a three hours jib and 90 minute pot life takes some planning. This
is especially true when you've signed up a buddy to help you for three
hours on his valuable Saturday morning. In those leading edge wing
skins, I put 4x8 plywood sheets on the grass, covered in 4mil poly
sheeting, supplied a hard roller on a broomstick, had my childbride
checked out in mixing and stirring (electric mixer) techniques,
pre=weighed 200g pots of resin-only 6 of them) and with all necessary
equipment at hand. This way, my buddy began rolling on the epoxy mix
into the cloth on the plywood outside while I started the
drymicro/flox/microslurry details inside, each of us howling demands
"for another 200g mix" to the metermaid. Weboth finished within minutes
of each other and jenny continued to feed mix, so we went out, grabbed
the end of the epoxy-soaked cloth and danced it into the waiting foam.
Within seconds, it was dropped into place and the fourhanded scraping
began. Instead of the four and a half hour exercise of the first wing,
this one took 58 minutes and was a neater job in the long run (and
lighter).
5. Close-outs: I found this to be the bane of my existence, for what
appeared at first to be a small area and thus simple, became a gargoyle
in the process! Until pre-wetting, I found my syrup hardening before the
cloth was properly sited, and desparation set in. How in hell does one
keep the ceiling plastered while the walls peel and the floor needs
scraping? I noted that the layer adheres better when the mix is
thickening, so prewetted TWO layers before assembly. By the time the
second was wetted, the first had thickened to the correct consistency to
stick in place while reaching for number two. Also, a slight delay after
microslurrying (prewetting time) seemed to guarantee better midterm
adhesion.
6. It was possible to have TOO MUCH epoxy in the closeout because one
hesitates to scrape excess out in the early layers for fear of moving
thin strips and yet minutes are ticking and everything is hardening
before the eyes. I then realised that the mixture (while thickening)
continues to flow inexorably for a long time ( a SLOW fluid). Having
seen the onsistent qualities of foam 'rubber' in applying equal pressure
throughout, I peelply the closeout, cover with poly sheet and then
insert precut pieces of foam to exert some insistence on pressing out
the excess. This is used to "goodge" out the sides which have been
precoated with protective tape - so the floor gets the excess. The foam
then gets urged to its task by woodsheet backing and weights/clamps. On
larger surfaces, I use ply backing held in place by a selection of
bungee cords. The result was quicker relief from the desparation of the
ticking clock, better 'wetting', faster cleanup, lighter closeouts and
less bubbles because of reduced time in process and constant squeeze. I
found I could do two in the time taken for one previously. All this if
you buy a sheet of 1' foam, and a roll of 2 or 4 (prefer the latter) mil
poly and precut.
7. I prepared a few devices in advance to accept lastminute excess epoxy
and thus redude weight. I have no proof for the need yet, but have
several dozen 1 sq ft 4 and 6ply layups for fabrication of doodads and
several feet of epoxy fairleads and feedthroughs for cables and lines in
the fuselage. The former came in handy making fittings for the wingtip
nav and /anticol lights.
8. I freely admit to wasting sheets of poly and foam, to say mothing of
pieces of cheap 1/4in plywood when speed and expensive components
prevail. There is a cost to space constraints in the middle of a layup
and I trade waste for space whenever granted choice. I'm only building
one........and I'm a slow learner. That's why I'm writing this - I
continue to hope there's another out there..... so I won't feel so
lonely.
9 One thing eludes me. I almost never get back to the 'leather' stage at
the correct time. It's either too sticky to cut the 'danglies' or it's
gone to granite while my back was turned. On the grounds that one
second's disturbance equals two hours of re-layup, I let it go (if it's
in position) and rasp it off later.
10. Two tools have ben invaluable. The first is a plane substitute
which goes by the name of Stanley SurForm which I understand is really
built under license from a Brown-Boveri design. It is the 'death-from-a-
thousand-cuts' sort of rasp surface attached to a handle like a wood
plane. This seems to stay deadly sharp for a long time and speeds up the
cutting of accurate edges to ailerons, wings and etc. The second is
newly-discovered surface for sanding gypsumwall - it's a fibreglass
flyscreen onto which is sintered particles of silicon-carbide grit in
various sizes according to speed of removal. Great for walls, but superb
for foam forming and finishing in finer grats. Look for same at the DIY
shop. Buy the three grit sizes for rough and smooth finishing.
11. Back up a bit - there is no question that for long straight edges
there seems little substitute for aluminum angle 'irons' - I settled on
the 1in by 3in by 12ft variety and was not disappointed. The 3in side
keeps the 1in side die-straight and the length satisfies the longest
run, the wing nose and trailing edges. ...and in the scurry at the end,
a series of quick-release clamps is really a boon!
I spose that does it for now, but felt I might add to the myriad items
of good advice to tyros received from so many others in the process, and
hope it helps someone.
Happy Landings
Ferg (#A064)
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