Nylon was the second man-made
fiber, coming just after the
discovery of the wood-based Rayon. Nylon was first made
from coal-tar. Nylon fibers absorb moisture and swell, then
shrink again as they dry. It's dimensional stablility
is environmentally influenced. The same skin that
is tight with afternoon sun will be loose and wrinkled on a cool,
foggy morning. This is sometimes called the "baggies". Nylon is
also considered to be UV sensitive
Water-based expansion and shrinkage is more or
less cyclical unless the fabric can be sealed. It's a subject
that has been discussed in the baidarka archives and it's a
method used by some builders today. Since the first seal-skin covers
were put on wet as an aid to fitting, you might stretch the point, and
say that shrink-drying nylon is an extension of traditional technique.
I found that dry cloth is no obstacle to good sewing,
nor is a warm room, as long as you sew your seams tightly.
Good stretch is the key.The tighter you sew nylon originally, the better.
The other thing when combating the baggies is to only use
8 oz. nylon. It works well and stays tight when properly sewn.
The heavier weaves are the ones that seem most
prone to the baggies, and I am not sure the extra density is needed.
I have never noticed 8 oz. nylon to be at any disadvantage
in the field for wear and tear, except perhaps for the
heaviest commercial service(rental boats).
Polyester (Dacron) is formaldhyde-based. It doesn't absorb
moisture the way nylon does and is supposed to be more UV
resistant. It is said to hold coatings, even acrylic housepaint, well.
Dyson's 9 oz. is a nice weave, it even has some bias stretch!
(not much of that in the 13 oz.). It sews and fits well. It doesn't
appear to be heat treated and it will shrink under a good, hot ironing.
13 oz. polyester makes a great skin but requires an "assertive attitude"
from the person doing the sewing.
Since Dacron's other name is permanent press, it will crease easily,
so handle with care. This also means you can make excellent folded seams
by using an iron set on "synthetic". It takes more heat, applied over a
longer time, to make cloth shrink than it does to iron it,, so it shouldn't
be hard to keep the two jobs separate.
(Overseas note: Builders have told me that import
duties in AUS and
NZ make cloth from the US too expensive. One correspondent is
looking into locally distributed sailcloth - the fiber is dacron and
the cloth comes in a variety of weights.)
Aircraft dacron is very promising, great
to work, with a nice hard finish,
like 13 oz. polyester. I've only used the heavy gauge, 3.4 oz. The approved
variety is pretty pricey. Not sure about long term durability since that is
something only time and accumulated abuse can tell. My supplier told me
that it was very UV sensitive. ( You don't see many translucent aircraft
paint-jobs, do you?
What about Cotton? Cotton canvas has a long history of
boat-covering, but I have no personal experience with it and so
am not qualified to judge. It tends to be heavy, prone to mildew
and not as strong or long-lived as the synthetics. It used to have
a price advantage, but that may have eroded. It is widely available
however. It shrinks when wet, which is a good trait for boatwork,
but requires a LOT of stretching.
The Role of Coating
Most builder's observations confirm that coating is an important part
of making the covering strong. Cloth weight, as in 9 oz. vs. 13oz. appears
to be less important. There is no point, however, in starting out with
an inferior substrate.
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