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Most of the builders seem to
classify and associate 'micro balloons' as if there is only one
type/product. In fact, the micro balloons (glass spheres) can be had in
varying wall thickness, all the way from the very thin that most are familiar
with, to actually solid spheres. The heavier the wall, the stronger they
are, harder to sand also. Hollow spheres can be had with specific
gravity ranges from .14 to .48, and can range from being able to stand
working pressures from 250 psi up to 3000 phi (not including and
contribution from the resin). A main advantage to using 'fillers' such as
glass spheres is for the purposes of reducing the resin quantity which in turn
reduces the expansion/contraction/shrink of material. So if you have to
repair an area that is to be finished and exposed, or fill in something, it is a
good idea to try and reduce the resin content as much as
possible.
Another product that I have
mentioned in the past is ceramic spheres. Ceramic spheres are usually in
the range of ~.8 specific gravity and have working pressures from ~1500
phi to 4000 phi. These are usually processed by being separated
out of fly ash by floatation. Again, they can be had at various wall
thicknesses/density and are stronger than the glass spheres. There seems
to be a better interfacial bond between the resin and the ceramic than with the
glass as well. The ceramic, while being somewhat heavier, definitely gives
a harder surface.
Personally, I prefer to use the
ceramic spheres, and instead of using the 'flox', which I believe are cotton
fibers, I prefer to use milled fibers. This is ground fiber glass fibers,
and can be purchased in various fiber lengths; such as 1/32", 1/16", 1/8".
I often use a combination of the ceramic spheres and milled fibers for
structural purposes.
Cabosil/fumed silica is often
misunderstood as being a 'light weight filler'. It is light in
dry bulk density only.
Gary R
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