As I was suggesting above, I was exploring various waterproofing oils and among these mixes is Penetrol  which can also be added to oil based paints.Floetrol is the acrylic version of Penetrol, and is" a latex paint additive that helps paint flow better, enabling you to paint without brush marks while allowing paint to adjust to difficult weather conditions. Also extends paint dry time to create faux finishing effects."

Both additives are made in Sweden I think.

Of course if you are papier mache-ing you don't want to be stuck up before you finish the job.This means that you work as many layers as you can before you step back to allow drying to occur. That's the way I've always worked .

I don't like working with oil paints anyway esp re smell, toxicity, difficult cleanup, recoating comcplications, etc so that's why I explored my acrylic options.

How strong and waterproof is this paper acrylic mix? I'm still doing tests. But obviously as an option to using standard papier mache glues, it works. Paint technology -- esp house paint technology -- is a big deal esp over the last 10 years and the ability of the paint to adhere to surfaces, even suspect surfaces, is seriously addressed in the lab and in situ.

I laid down maybe ten layers of Kraft paper and now have a convex frisbee  that flexes (more than I wanted). So I'll sand back the top layers and lay down some more paper  to see what impact more coatings have on stiffness.I'll also need to explore other materials in the mix such as canvas, linen & cotton. (I'm thinking that I'd crisscross strips of these materials to see  if (a) they adhere and (b) they strengthen and (c) give the shape rigidity.

I just did a test of a very simple paper mache and polyurethane coating for waterproofing. I made a half sphere of paper mache (flour/water/newspaper strips) by coating a balloon and letting it dry. Then I coated this with two coats of simple polyurethane (the clear stuff that you would usually use on a hardwood floor). Only the outside was coated (so that I could check the paper mache interior for water seepage.

After drying I put it on a bucket of water and placed weight on it to simulate a boat's immersion in water. I checked it every couple of minutes for one hour and ended the experiment, declaring it a huge success.

Things to keep in mind when building a surface to waterproof: keep the paper layers and cuts of paper smaller than you think, I used pinky finger width strips of paper mache in order to eliminate possible gaps and openings for water to enter. When coating, apply the polyurethane liberally even if it means letting it drip off the sides. This will ensure that an adequate amount will not only coat the mache, but soak into the paper creating a better seal.

I am not certain as to the true time period that my paper mache would remain waterproof but I can say that the interior was bone dry after one hour of sitting in water which in my mind is an excellent place to start if one were looking for a full scale boat. The next test I will conduct is for a half sphere painted with latex paint and then coated with polyurethane.  Naturally a painted boat would look nicer and I want to see if the latex interacts poorly, or if one needs to follow a specific order of coating to achieve maximum effective water resistance (ie.. paint over poly, or poly over paint? )

Below is a very short video of the testing:

http://www.youtube.com/watch?v=E3UeY4CJHKo

Nice to see some experimenting going on. Always fun to do, I think.