In order to get a fabric which weighs 12 ounces for every square yard you would need to use yarns which are twice as dense as those used for a fabric which weighs 6 ounces per square yard.

For a heavier weight (stronger) fabric they go to a much thicker yarn, which has to be woven slightly looser. This gives you deeper wells between the areas where the yarns cross in the weave. These deeper wells need more coats of resin to fill them, which adds up to time, money and weight.

Strength can be related to the amount of glass over a certain area. With the thicker, more coarsely woven fabrics you need a lot more resin to get a smooth surface. That adds weight without adding strength.

With the thinner lighterweight fabrics youcan get a smooth surface with less resin.

If you use multiple layers of lighter-weight glass cloth you will use more running yards of cloth. At some point you may get a discount for buying more than a certain number of yards of fabric. If you want 12 ounces of glass on some icebreaker, you can put on 1 layer of 12 ounce, two layers of 6 ounce, three layers of 4 ounce, or four layers of 3 ounce fabric. Compare the corresponding prices for all these materials. In a rational world the heavier cloth would be twice the price of a fabri with half the weight. 'Tain't so! You might use 12 yards of 12 ounce, or 24 yards of 6 ounce -- but your supplier might give you a better price for buying 25 yards of the 6 ounce fabric.

6 and 4 ounce fabrics are popular weights, so there is a greater variety of widths for these fabrics. You might save more money by buying a shorter, but wider piece. A few minutes with a calculator, and a rough sketch of how you are cutting your fabric, can save you some money.

I agree with another poster that 12 ounce fiberglass fabric is not needed for covering wood kayaks. I might consider something like that (or even heavier material) for a skin for a skin-on-frame kayak, though. 4 ounce and 6 ounce seem to be the most popular weights. Unless you expect serious scratching (rocky landings) anything more than two layers of 6 ounce strikes me as overkill. 4 ounce seems sufficient for our uses, and two layers (total) of that weight on the center of the bottom of the hull are great for ding and dent resistance, if you really need it. Otherwise, 4 ounce will hold your boat together nicely, it may cost less to buy, and it needs less resin.

The glass tapes are woven to the width you buy them. The length can be very great, but the fibers just go so far to the left and right before they come to the last strand on the loom, which the wrap around as they get woven back and forth. This woven edge is called a "selvage" edge on your fabric. Tpaes which are woven this way have the threads going straight up and down, and straight left and right. They are good and strong, but less flexible to bend into curving spots. You have to sand down the woven edge.

By contrast, you can cut strips of your glass cloth either in this linear fashion, or, you can cut strips at a 45 degree angle to the length of the fabric. When you cut it this way, which is called "Bias" cut, the strips you produce will be short sections, maybe 4 to 5 feet long, with the fibers running at 45 degrees to the length of the strip. This orientation of the yarns and fibers allows the strip to be bent around curves more easily, and is preferred.

If you handle the bias-cut strips very little after you cut them you should not have a problem with them fraying. Applying resin with a squeegee will cause them to splay out a bit at the edge, but this actually helps you when you go to feather in the edges by making the edges effectively thinner. Any loose fibers that may d up are rapidly tamed with a scraper when the epoxy is set, but still a bit soft (it is at its "green" stage) or a day later you can remove them with sandpaper.

Hope this helps.

PGJ