On my Clarke I spread beeswax salad-bowl finish on the back face of the wood block, to keep it from accumulating gunk. (Left the windway totally alone.) It doesn't seem to have hurt -- only the Clarke takes a lot more wind than I have. For now I'm sticking almost exclusively to the Gens and Megs.
You can reduce that wind requirement by pressing the metal top of the windway down, narrowing the space. Do this very gradually-- thumb pressure should be sufficient. Press a tiny bit, then play for a while, repeat until it plays the way you want. It's still a pretty airy whistle, but you can probably improve it significantly.