A few more words of wisdom and experience from Gary Hunter that I thought
worth passing on. There's no substitute for experience, even better if it's
someone elses?!?
Graham
> Is Nat's statement that if ... the glass wets out and goes transparent
> > (I'll be using MGS epoxy) it's OK, and that if it stays white it isn't
>wetting out and is no good ... a reliable test? It makes sense to me, but
>you're
>the expert. Please help me out here .... Jim S.
>
>As usual, Nat is pretty much correct. (Nat Puffer, CoZ designer)
>
>Glass fibers have a "SIZING" applied to them when they are made. Most
>resins do not inherently wet out or adhere to the glass fibers very well by
>themselves. Primarily because of a disparity in their surface tensions.
>The sizing is applied to the fibers as a very very dilute dispersion or
>solution in water. The sizing solution does several things. The water
>(introduced as a fog or spray) is used to cool the fiber from its molten
>state. It is also the carrier solvent for the sizing. The solution is
>also a lubricant for the downstream processing of the fiber up until it is
>put on a spool and dried in an oven. Most importantly, the sizing
>reduces the disparity in surface tensions between the glass fiber and the
>resin. The sizing formulation is tailored for the specific kind of
>"RESIN" that will be used to wet out the glass fibers. A polyester (boat
>resin) requires a different kind of sizing than an EPOXY resin. There
>are some sizings designed to be "dual" compatible. The only way know
>what you have is make sure you buy the fiberglass specifically called out
>by the designer of the aircraft.
>
>Most of fiberglass you buy at the auto parts store, or marine supply house
>is intended for polyester resins. However, with West Systems Epoxy being
>sold through many auto and marine supply stores for auto body and boat
>repair, most of the fiberglass is probably dual compatible. You can
>probably use that fiberglass to make or repair your wheel pants or fairings
>and such. But, under no circumstances should you consider that glass
>source for structural components of your aircraft.
>
>One way you can tell if the fabric sizing is compatible with he resin you
>are using is a visual indication of "wet out". Just like Nat said. If it
>doesn't wet out the glass fabric, it doesn't turn transparent or
>translucent . It stays whitish.
>
>Since these sizings are introduced to the glass fibers as a water solution,
>they generally contain an extremely minute amount of surfactants to keep
>them dispersed in water while they are being applied to the fibers. These
>surfactants remain on the fiber after the water is dried. As such, if
>water is re-introduced to the glass fabric, the sizing can be washed off or
>moved about. You have probably noticed that if you had accidently dropped
>a bead of sweat onto you glass fabric, it makes a permanent mark in your
>laminate, even if you allowed it to dry before you did the lay up. This is
>because the sizing was washed off the fiber in that particular area.
>Similar results can occur if the entire roll of fiberglass were to become
>wetted.
>
>DO NOT USE glass that has become wet.
>
>Exposure to high humidity generally doesn't hurt the fiberglass like direct
>contact with water. However, it can run the risk of mildew forming on the
>fabric. Generally, this will only occur on the first layers of the roll.
>Further down in the roll, the glass fabric is probably OK.
>
>SO, if you think the glass is suspect, throw away the first few layers, and
>check the fabric below before using it.
>
>Properly stored, the fiberglass we use should have an infinite shelf life.
>I ALWAY store my fiberglass wrapped up in polyethylene plastic to protect
>it from contamination of all sorts. A 3 or 5 mil polyethylene painters
>tarp, rolled around the roll several times. Make sure the polyethylene
>plastic is about 1 foot wider than the roll. Tuck the ends of the
>polyethylene plastic into the ends of cardboard roll, and wedge it into
>place with a mixing cup. This is very handy, and eliminates hassles with
>masking tape and stuff.
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