: . . .Just to be clear, when you talk about a clear urethane, are you are
: referring to a varnish?

Well, in a manner of speaking, yes. But not exactly. I'm referring to a clear liquid coating which dries to a hard flexible film. "Varnish" is both a general term, and a specfic description of a type of coating made by dissolving natural resins in a petroleum-based solvent, usually with the addition of some linseed oil. There are products around which are based on urethane or polyurethane molecules which are sold as "varnish" because they do the same job. Or at least they claim to do the same job. In many cases they do a much better job than the traditional varnishes. I like to try and distinguish between the three chemical bases (polyurethane, urethane, and resin) because otherwise you will find people trying to call lacquers and shellacs "varnish", too. Shellac, of course is an alcohol-based finish, while lacquer is nitrocellulose dissolved in a very-fast-evaporating solvent like ether. Some of the urethanes and polyurethanes are prepared from two components just before use (just like mixing epoxy resin with epoxy hardener) and harden like epoxy, too, by polymerization, while the other finishes need to have their solvents evaporate. If I were making a grandfather clock I could use any of these finishes to protect the wood. But for marine use the shellac and lacquer would not hold up. On the other hand various formulations of varnish have done well for many decades--and you can buy brands which are essentially unchanged for over 100 years. Types designed for outdoor use are frequently called "spar" varnishes. But the real stars are the urethanes and polyurethanes. They dry faster and give a harder, more durable finish. I've seen brands that describe themselves as "Spar urethane" and "urethane spar varnish", To my mind these descriptions are attempts to appeal to people who only know the term "varnish".

A urethane or polyurethane-based clear material cna be used instead of varnish. It bonds well to the fabrics, soaking into the weave and hardening. The same base can be mixed with a pigment and be called "paint", or "enamel". The added pigment shouldn't affect how well it grips the fabric.

Boat enamels have been used on cotton canvas-covered canoes for well over 100 years. When polyester and 50/50 poly/cotton fabrics were woven into canvas they still worked fine. As I understand it, some sailboats have canvas glued to their decks to give better traction and to protect the wood. This canvas gets painted with "topsides" boat enamels, and this has been done for years. I suspect that when a paint rep says he is not sure he can recommend any of his products for painting a kayak he is probably parroting the company's lawyers.

If you figure the total number of square inches of surface area which is in contact with the water, and divide that into the total weight of boat, paddler and gear, you get something like 0.3 pounds per square inch of pressure on the hull. Just about any paint film can support that. Some are strong enough to support over 100 times that. (over 30 psi). The pressure, of course, is mostly borne by the fabric. If you put latex on fabric it is very strong. Early tires were made that way. But under higher pressure latex alone would deform, and stretch like a balloon--which is why a lot of balloons are made from latex.

We don't use natural latex to seal the boat hulls because it doesn't age well. But lots of hulls (and diver's suits) were covered with vulcanized natural rubber 100 years ago. Then as synthetic rubber compounds were developed around the 1940s we got a wide range of useful materials. George Dyson sells neoprene and Hypalon (r) for coating baidarka skins. Hypalon-coated fabrics are the top choice for making inflatable boats that are used for running rapids. Foamed neoprene is used for wetsuits and drysuits. I've been enthused by polymeric rubber roof coatings. You can buy it at most hardware stores. They are cheap ($30/gallon or less) and strong, and highly UV resistant, but they need some protection from abrasion.

: . . .So for a proper sealing and smooth,
: gloss finish I am looking at two coats of varnish followed by
: two coats of topside paint?

Generally people will put on 2 or more coats of a urethane or polyurethane finish. Conventional varnishes can be used, but those can take weeks (and sometimes months) to harden fully when they are applied thick. With rel varnish the boat is usable a few days after the coating is applied, but the coating gets better until it finally cures all the way. The urethanes cure faster. Urethane-based coatings used for painting cars, for example, can be baked to harden them in a few hours.

There are a few "interior" urethane finishes which are a disaster to use on polyester fabric. They will flake off after a few hours on the water. Oil-based, exterior urethanes are generally good. Water-borne exterior urethanes seemto be OK, but I'd buy a small can and test it before committing to an entire hull. I don't know what type of fabric you'll be using, or if it has been treated in any way by the manufacturer. There may be dyes, starches, stain repellents, permanent-press, wrinkle-release, or stain-release chemicals coating your particular fabric. These can affecthow well any given coating will work. With old fashioned 100-percent cotton canvas there wasn't that much of a problen! So, test a sample of scrap fabric.

: I work in a boat yard and have checked with the factory reps for
: the various marine finish companies. Their response was that
: they could not recommend any of their products as they had no
: experience in how it would adhere to fabric or what the stretch
: properties are. Thus the real world experiences shared here are
: invaluable.

In general, SOF builders have had good results with all the topline boat enamels. They all seem to bond well to the fabrics and are flexible enough for our purposes. Exterior housepaint has been used successfully, too. (Both oil-based and latex). In your case why not try two or three thin coats of (Yellow is it?) boat paint and check for pinhole leaks. Probably won't find many. Every year you can add a coat of enamel or cover it with a clear urethane or varnish--or leave it alone. After a decade you can report back on how durable your method was.

Good luck with your project.

PGJ