Mike,
For what it's worth, if I'm adding a bottom board, tailboard, or sprig, I'm thinking protection it affords to the cork + its overall strength and
rigidity.
Your example of a "sprig" made from PVC is a good one. A properly bedded, pre-bent but rigid PVC spig set deep into a pinnie drake is far stronger than any I could insert that was made out of hardwood. To my thinking, the strength and rigidity become even more important when adding a more delicate piece like those involved in simulating a sprig or long tail. Flexible materials would seem to be a solution, but the reality for me has been what transport in pocket bags will be like, and what that flexion does to paint in terms of adherence and cracking. The material may flex beautifully, but the paint won't.
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I agree that a PVC tailboard would be strong, but here again, I like to carve that "tented" shape that is natural to a duck's tail into a tailboard, so the best soulution has been wood stock of a sufficient thickness to do so. Finding PVC carvable / grindable material of sufficient thickness would be the trick and overkill on a normal tail, as the wood is sufficient. When pegged through the bottom board with a hardwood dowel, it easily meets the protection + strength + rigidity qualities that I'm looking to have.
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Black cork can be a "bear" when it comes to tailboards. Best to stay thin and flat with a minimal slot-cut and plenty of material left to surround the board- as well as some thought given to what pattern shape on the tail end best accomplishes that. I used to use a slurry of Val-Oil and black cork dust to fill larger voids before sealing. Nevertheless black cork birds always seem to be riding lower in the water by seasons end, regardless of how well they get sealed ;-). Total immersion in a bucket of sealer is probably called for. I carved a rig of 22 magnum - sized, black cork decoys in the late 70's. All those birds had a circular block of solid cedar under the heads that ran all the way to the bottom board + a dowel pegging the whole shebang together - not so the tails. Traded one to Gene Chandler to reimburse him for gas on a scullboat delivery, then sold the entire rig to some collector from Michigan at the old Stillwater Show in about 1985.. The only one I kept got sawed off for this shop sign (...probably had a broken tail ;-):
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