Tuesday 4 May 2010: I've been slathering coats of Target 9300 water-based polyurethane coating on the skin to waterproof it. First doing the hull, then an hour-and-a-half or two later flipping it and doing the deck. This coating has been used by David Mills, aka "stumpy" who posts regularly on the Blue Heron Kayaks' Kayak Builders' bulletin board for some years. David has been using this on nylon fabric. I was surprised to hear that because everything I have heard about nylon is that most things won't stick to it, or will be too inelastic and subject to failure when the nylon stretches beyond the limits of the coating.
By the way, if it seems like I am growing less enthusiastic about updating this blog, it is mainly because the editing interface is terrible, and especially the process of inserting these images, and linking them back to my website is an enormous pain-in-the-ass. I find it so frustrating to manage that I am ready to consider that finding myself in a situation where it might be necessary to resort to cannibalism to in order to survive, would be preferable to finding myself in a situation where it is necessary to create or edit posts using this crappy web interface! That's all I'll say about this right now because it has worn me down and I am tired of fighting with it....
Anyway, where was I? A nice long thread was started on Blue Heron's Kayak building forum about fabric for skin-on-frame kayaks. This thread rambled along for awhile and contains a lot of interesting information about fabric choice for skinning. But eventually David gives a nice explanation of the specific water-based polyurethane sealer he has been using for a few years; and how he applies it. Did I mention that there are many who would advise against use of water-based poly? The experience of some who have tried is that it would get soft and turn whitish when it gets wet; or that it might even just peel off in sheets. The only two references I have for positive results using water-based poly come from Robert Morris, in his book, "Building Skin-on-Frame Boats", and from posts on the Qayaq USA bulletin board by Turner Wilson. Robert Morris mentions that an interesting characteristic of water-based poly when used as a sealant, is that it does not stick to wax. So that by waxing the frame, it is possible to prevent the skin from becoming glued to the frame by the sealant. Probably I am crazy, but this idea appealed to me. Just noticing, how springy and alive the lashed frame is when it is bare, made me interested in trying to preserve that feel in the boat after it has been skinned. So the frame had been oiled with tung oil and then waxed with some Meguires paste wax before skinning.
Another strong motivation for my interest in water-based polyurethane is a desire to find a boat building technique that is less toxic, and that generates less waste. This was a big part of what motivated my interest in skin-on-frame boats.
The Target water-based polyurethane was very easy to use. Since it is water-based, it is very low on volatile organic compounds. Everything can be cleaned up with water. There are no volatile thinners or solvents required. The same brush can be used throughout so there is much less waste. It is very much like working with acrylic paint. In fact, ordinary acrylic paints can be used to tint the Target coating. David Mills has some very nice examples of whimsically painted boats colored in this manner. In fact, I mixed a little titanium dioxide into the layers that I put on the deck. I did that to make it a little more reflective in hopes that would help to keep it cooler in hot weather.
There is a specific technique for using the Target coating successfully that was developed by David in consultation with Target. The technique is to use coating that has been thinned with water to a concentration of 50%. Special care is taken with the first two coats using the thinned sealer, to work the coating into the weave of the fabric as thoroughly as possible. Since David has been using this coating on nylon, that instruction is likely especially important in that case. With the rather loosely woven polyester fabric I am using, it is quite easy to work the coating into the weave. I followed the same procedure, using two initial coats of coating thinned 50%. After that I just slathered on 6 or 7 coats and used the good part of a gallon! I laid down the rice-paper ensos onto the bow with the first coat of full-strength coating.
The second instruction to be observed, is to allow the coating to cure for 72 hours before exposing it to water.
In the picture below, you can see some of the pull-holes that opened up while I was sewing this very loosely-woven, 8 oz. polyester cloth. Over many coats (6 or 7) the Target coating eventually filled these holes right up. You can see how the boat is leaning to one side on the starboard bottom panel and so I can apply a very thick coat of polyurethane and allow it to run up against the center seam which traps it where it can fill in the holes. Neat. I'd like to type some more stuff after this picture, but this crappy editor makes it impossible to get a cursor down there without deleting the image and reloading it. This makes me sad.