Bob,
Looks like my first attempt at an answer got eaten by my new "computer" (it's a palm pilot with wi-fi ) setup. I'll try again:

Don't worry about adding any extra layers of fabric now. Finish the boat and get it into the water.
Paddle it for a few weeks before you varnish it.

You should develop a pattern of fine scratches from th normal wear and use of the boat.If the scratches are very light you can buff them out before you varnish.

If ome of the scratches are deep--deep enough o need to be sanded smooth-- then do that, and fill the depressions with some fres epoxy before varnishing.

If you have scrates and dings whih go through the glass, patch them.

If you have a large area with a lot of abuse--something worthy o extensive patching--then cut a large piece of glass cloth and cover over that area. Let th scratch pattern show you what needs extra protection, and don't waste time beefing up spots that get negligible abuse.

Each year you'll add more experience--in the form of bumps, scratches, and dents-- to your boat. I generally recommend leaving he repair of these until the end of the paddling year. At that time the hull gets a lght sanding to remove the old varnish, patches are epoxied overthe major damaged areas, and the boat is sanded smoth and revarnished. With regular varnish the long wait in storage lets the varnish harden so the boat is ready for launch first thing in the spring. With the polyurethane coatings you don't need such a long wait, but all these coatings (including the epoxy) will get a bit tougher with some age

It is possible that you may never need ot add an additional layer of glass cloth to your boat. The 6 ounce glass is pretty tough stuff. It is also relatively heavy, and absorbs more than it's weight in epoxy resin. If you are planning regularly to bash through some rocky rivers, then added layers might be in order. But for a kayak used on water over 12 inches deep you'll get very little abuse to justify the extra weight and cost.

Hope this helps

PGJ