The e-aa hotline post today contains a short video of spreading "dry
micro" to finish a glass layup. His mixing example was good but his
finish on spreading the micro left a bit to be desired. He had a very
small patch of glass for his example and he just "muddied" the micro on
to finish the video. Dry micro is nothing more than "body puddy" for a
finish coat, fill the weave and pinholes for a smooth finish.
Anyway, that reminded me of a process that hasn't been mentioned on the
net for a good number of years and would be beneficial to those
building. It is called "hard shelling" and makes glassing without a
helper go much smoother. As Langford and others have demonstrated on
glass layups on foam, you first apply a wet micro slurry to fill the
foam and then follow with the glass layup. This eliminates filling the
foam with the heavier straight epoxy. This can rush the layup when doing
a large area such as the wing or even tail surfaces. To make it easier
for one person to do the glass layups there is a process called "hard
shelling".
First, mix and spread the micro slurry on the foam and let it cure.
Hence the term "hard shelling". When cured, brush the surface a time or
two with sand paper to eliminate any prickles that tend to snag the
glass when doing the wet layup. Now do your wet layup with glass and
epoxy without having to do the micro slurry step in the process. You
will end up with the same glass to foam adhesion whether doing it in one
step or two but the two step is much easier when doing it with one
person. I did both wings, top and bottom, as well as the tail surfaces
using the "hard shell" method and did not feel rushed at any time.
Some might argue that the single step process is better but my reasoning
is this. The slurry to foam bond is the same either way. The epoxy
bond with the glass to the "hard shell" is greater in strength then the
strength of the foam. If you peel off a glass layer using either method
your get the same results. The separation is the foam at the slurry
line with either method. It will never separate at the epoxy to "hard
shell" line.
I did not develop this method. I got it from a fiber glassing book some
25 years ago and it is quite an acceptable method as proven with my KR.
As always, YRMV...........................................
Another tip: If you get the foam done right you won't need a lot of
"fill", only enough to fill the weave of the glass. I used a layer of
"deck cloth", 1.2 oz glass to cover the KR clothe and had almost no fill
at all. I also used it to cover the ply on the fuselage. Again, I got
that tip from "what's his name" that wrote the four books on
experimental aircraft construction, Oh yea, Tony Bingalis.
Larry Flesner
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