There are many kinds of Polyurethane which work for a SOF kayak and even more kinds that don’t work. You need to look out for certain properties and I will try to list some challenges here. The fact I am even trying to is because I have miserably failed on a number of occasions. This time with the mix. From the top of my head I used 1:4 as a formula when I should have used 1:1,5. The pictures you see below are before I knew everything was going to h**ell. Even further down you can see what I did about it.
Skin on frame kayak coating properties
- Dry to the touch within 4h is nice.
- Possibility to apply wet-on-wet. Or you can wait a day inbetween 7 coats of runny, stinking mess in your workshop.
- Curing and repainting within 24h. Fully cured within a week. Some PU’s can be repainted if you sand them down, but generally once it’s cured, nothing sticks to it well.
- Potlife. No less than 15 min. 30 or more minutes would be ideal. Your PU has to have a chance to penetrate the fabric before curing. If your PU has high viscosity it will just act as a layer on top of your fabric and can be peeled off. See images below where the fabric is white.
- No solvents released into the air if possible. You only have one set of lungs.
- The PU stretching at least 30% is good on impact
- No softer than 80 Shore A, no harder than (something like) 50 Shore D. Search the net for info! Your coating needs to be pretty hard but flexible. A hard fingernail should be able to make a scratch but not more.
- You basically need a concrete floor sealant (two component) PU or a boat use equivalent.
- Your PU needs to penetrate the cloth so depending on fabric composition, use different viscosity PU.
- The PU and “skin” weigh between a third and half of your wooden frame. More layers make your kayak more durable but also heavier. A heavier skin does the same of course but can also soak up more PU and get even heavier. If durability is your thing, go for it! You still probably end up with the lightest sea kayak you’ve paddled.
- A skin on frame kayak can take as much or more beating than a glass fibre kayak but is not as durable when exposed to repeated abrasion.
I had previously realised my mixing mistake and took a chance by applying a properly mixed layer on top.
And then, when I came back from a paddling trip a week later from Åland… that layer had shrunk and the uncured bottom layer had released from the fabric into a wrinkly mess!