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Dear Marv:
This ought to confuse the topic threads, but I figured one letter for
multiple topics since brevity is the soul of wit.
Solvents
I read the comment that acetone has a specific gravity of 0.7 and so it
acetone vapor would not collect on the floor (like propane and other
hydrocarbon fumes). This sent my off to the books to check my memory.
Acetone is lighter than water (specific gravity of 1.0 vs. 0.7 for acetone)
but it has a formula of CH3COOH giving it a molecular weight of about 60. This
yields a vapor density roughly twice that of air at the same temperature.
What this means is the vapor is heavy enough to collect around your feet in
concentrations high enough to light off, so BE CAREFUL WITH ACETONE. Natural
gas (mostly methane) and hydrogen drift off, but not the hydrocarbons.
Sanding prior to bonding
An earlier writer got it right - clean the raw surface BEFORE sanding. If
you sand an uncleaned surface, you transfer the crud to the sandpaper and then
contaminate the surfaces you sand (although one would expect the contaminants
to be mostly gone after a good bit of scuffing). The corollary is use clean
sandpaper to keep the bond area clean. I picked up an additional tidbit at a
seminar put on by a firm that repairs composite parts for air transport class
aircraft. Bond within 4 hours of sanding. The sanding creates a "high
energy" surface that is more amenable to bonding than one that has sat round a
long time and adsorbed water vapor. So the sequence is: solvent wash, sand
(clean sand paper), warm gently (drive the residual solvent that did not
originally evaporate out of the core and pin holes) then bond within 4 hours.
This provides the "ultimate." Now having said this, be advised that the bond
lines in our planes have been designed with MASSIVE safety factors, huge
contact areas, and presumptions of lots of air bubbles in addition to the
normal increased safety factors that the FAA requires for composites (2:1
instead of 1.5:1 for normal category metal airplanes, load to "initial
yield"). If you want to build to the limit using composites you need
stringent quality control, controlled environments, autoclave (high pressure
high temperature) curing, and continuing checks of materials and processes.
Since we don't do these things in our garages, large safety factors are used
instead. That is one reason why composites do not end up as light as they
could when in the hands of the homebuilder.
Epoxy pumps
I am using the ugly Shell Epon DPL 862 and TETA system. I have checked my
sticky stuff pump on numerous occasions and found it to be within about 5%
most of the time. When I made a batch that did not set up as rapidly as prior
batches, I would check again. I found that the check valve in the TETA tub
would collect air which I could release by poking it open by reaching down
through the TETA and popping open the ball in the check valve using a plastic
chop stick. (TETA oxidizes most metals.) By popping the ball open a few
times I could get bubbles to come out. When they stopped coming out, I would
recheck and the ratios were back in line. So now I check for air with my chop
stick every time.
When I had a non-critical bond line that did not cure fully hard as fast as I
normally expect, I found that heat finished the job. I use a forced air
electric space heater directed at the region. About 140F for 6-8 hours will
usually get it fully hard for sanding and finishing without gumming up sand
paper. Small parts I put in the oven at 140-160F (after initially hardened)
just because I get impatient. Don't get too impatient and heat it while still
a little soft. Bubble city. (Been there, done that.)
Water based paints
I primed my wings with the System 8 white primer with satisfactory results,
but have since switched over the Poly-fiber Smooth Prime and Silver coat
system since then. I spent some time at OSH with the Poly-fiber people, and I
became a convert. Forget everything you used to know about painting. (Hard
for me - I have done some car restorations.) Follow the Poly-fiber manual
which is well written. They show you how to spray the primers if you insist,
but don't bother. Roll it on as instructed. It initially looks like hell,
and you are sure you must be doing it wrong. Then sand. This is where the
payoff is. It sands beautifully, quickly, and easily. I use 120 grit for the
big drips, 240 for most of the balance, and 320 (their recommended grit) for
the final pass. You can roll on three coats of primer in one day, sand the
next day, roll on the silver UV protector that evening, and continue the
following day. I have not used the finish coat yet, but I believe that their
approach (no clear coat, sand and buff the final finish coat which is
toughened with additional cross linker) is probably the way to go for non-
metallic finishes.
I agree with other writers: their feather weight epoxy beats micro hands
down. One way to ease application is to spread it on as best as you can, then
cover with strips of plastic film, then roll it out (like using wax paper and
cookie dough) to get the final surface more uniform. Multiple narrow strips
of plastic film allow you to work around complex curvatures. It gets rid of
the bumps and valleys and reduces sanding and secondary filling.
With regard to rain damage on leading edges, Lancairs (especially Lancair
IVs) go fast enough that the rain will damage leading edges unless you use the
jet grade of polyurethanes which are extremely tough and not recommended for
homebuilder application. I know the PPG automotive stuff, good as it is, will
not stand up to the rain for more than a few minutes before it starts making
holes at 300 KTAS. (As I recall, impact damage potential for liquid droplets
goes like the speed to the fourth or fifth power so speed counts a lot. Rain
is hell on reentry vehicles.) The only solution I know available to us that
is really effective is polyurethane leading edge tape. Put it on, accept the
5 knot speed loss, and be done with it. If the five knots is intolerable, the
3M guy at OSH said one could put masking tape on the leading edge tape up to
the trailing edge, another run of masking tape 1/2 inch behind the trailing
edge of the leading edge tape, and then hand paint some clear coat in the
space between masked regions. While still wet, pull off the tape so you have
a transition between the leading edge tape and the wing surface, and finish
sand and buff to get a good taper that eliminates the step arising from the
tape thickness. The result is smooth enough not to trip the boundary layer
and maintain the laminar flow (or so he said).
Unless you like to paint leading edges several times per rainy season, use
leading edge tape. Don't forget the vertical stabilizer. And accept that the
rings around your air inlets on your cowl will probably get some acne when you
fly in the rain. Or you could slow down (forget I said that....)
Fred Moreno
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