I just finished building a Sea Duck with a few modifications. I have a similar size plane that use a modified "swap-able" power pod, so I modified the nacelles to accept the power pods I had. I made them taller to allow the power pod to slip in above the wing spar while still being removable. I went with a scratch build and used DTFB but wanted it waterproof to fly off of snow and at the family lake place this summer. I did a fair amount of experimenting during the construction.
My experiments/results.
1. Brushing polycrylic directly onto DTFB Paper.
Result: Appeared to soak into paper well and seal without lifting paper off foam until water was applied. the water soaked into paper and the paper lost its structural integrity.
2. building as normal with dtfb then removing paper and brushing on polycrylic and applying new craft paper covering.
Result: this was a slow, tedious process and was difficult to get into all the tight corners. I have not tested on water yet but suspect water may weaken joints underneath/inside the areas were the craft paper was applied.
3. remove paper from whole sheet and cover entire sheet with polycrylic/craft paper.
Result: this seems to be the most effective way to waterproof the pieces. there is a significant amount of warping in the sheet when drying, even when weighted flat. with the design of the sea duck, the bends/internal braces seemed to have taken out most of the curve from this process.
Conclusion: I would recommend buying a sea duck kit from FT and not scratch building from DTFB if planning on flying on snow or water. I probably have nearly the same cost into materials as I would have if I had just bought the kit.
It flew pretty well even though I forgot to check the CG. The craft paper coating also doesn’t seem to have the same crush resistance. A low speed crash into a chair during a botched landing left a sizable dent in the leading edge of the wing.