"Lew
Polyester/roving produces a rather brittle, heavy laminate.
Epoxy/knitted glass produces a more ductile, lighter weight laminate.
Pound for pound, polyester and woven roving cost less per pound, but
you use more pounds, so the cost differences tend to disappear.
Epoxy, knitted glass and a 1/2" Airex foam core would produce a very
light weight, bullet proof, dinghy or kayak.
Just remember three things.

1)You will buy a commercial dinghy for less money than you will spend
to build one.
2)You build one because you want to do it.
3)Once you use epoxy and knitted glass, you will never consider
polyester again.


Lew is right on every particular.

Lew is right on every particular.

Yes... but. Lew's Knitted glass - when bought in 'roll' quantites is
about a dollar-a-pound more expensive than woven roving. Traditional
thinking is that woven fabric is used primarily with fiberglass 'mat'
in a polyester layup, one between the other. The basic reason for
this is that the woven, i.e. "over, under, repeat" nature of the w.r.
causes "hollows" where the 'high' strands cross-over the 'low' strands
of glass. The mat (in addition to providing significant bulk, and
sucking up a lot of resin) is thought to "tie" the woven layers
together, by bridging these 'hollows.'

Now, in a large epoxy resin layup one can significantly reduce the
cost, while maintaining adequate strength by continuing to use woven-
roving. The mat is forsaken in favor of a slurry of milled fibers and
a little cab-o-sil mixed to a 'light cream' consistency. That is,
layer of w.r. wet out with straight epoxy. Squeege. Trowel in a
slurry of epoxy / milled fibers / cab-o-sil, then next layer of w.r.
Finish wet-out with straight epoxy. Squeege. Repeat.

The milled fiber + cab-o-sil slurry is filling the hollows and
providing a tie for the next layer of woven fabric. A 10 lb bag of
cab-o-sil is like way cheap, and will last the entire project. A 50
lb bag of milled fibers is similarly inexpensive.

And, becuase we've eliminated the mat, resin-to-glass ratio's of 50 /
50 are easily achived.

So, in a large project where cost is a concern my belief is that one
ought not write-off woven roving as being a suitable material.

MW

wrote:

Lew is right on every particular.

Yes... but. Lew's Knitted glass - when bought in 'roll' quantites
is
about a dollar-a-pound more expensive than woven roving.
Traditional
thinking is that woven fabric is used primarily with fiberglass
'mat'
in a polyester layup, one between the other. The basic reason for
this is that the woven, i.e. "over, under, repeat" nature of the
w.r.
causes "hollows" where the 'high' strands cross-over the 'low'
strands
of glass. The mat (in addition to providing significant bulk, and
sucking up a lot of resin) is thought to "tie" the woven layers
together, by bridging these 'hollows.'

Now, in a large epoxy resin layup one can significantly reduce the
cost, while maintaining adequate strength by continuing to use
woven-
roving. The mat is forsaken in favor of a slurry of milled fibers
and
a little cab-o-sil mixed to a 'light cream' consistency. That is,
layer of w.r. wet out with straight epoxy. Squeege. Trowel in a
slurry of epoxy / milled fibers / cab-o-sil, then next layer of w.r.
Finish wet-out with straight epoxy. Squeege. Repeat.

The milled fiber + cab-o-sil slurry is filling the hollows and
providing a tie for the next layer of woven fabric. A 10 lb bag of
cab-o-sil is like way cheap, and will last the entire project. A 50
lb bag of milled fibers is similarly inexpensive.

And, becuase we've eliminated the mat, resin-to-glass ratio's of 50
/
50 are easily achived.

So, in a large project where cost is a concern my belief is that one
ought not write-off woven roving as being a suitable material.

About the only thing missing from the above is a little spit and some
baling wire.

Lew