Don't use anything. Doping the wood prior to covering allows you to fine sand the wood to get rid of the "fuzzies", and some builders just use dope to apply the Koverall. I find it a tad messier than Stix-It. Just fine sand the airframe where the covering will contact the wood (about 360-400 grit), and then just paint the Stix-It on and give it about 5 minutes to set, then apply the fabric and tack it into place.

I only apply Stix-It where I will have an overlapped seam. On the wings, that would be the trailing edge, wingtip to the L.E. and the aileron bay. I try to use a single piece for each wing half, to avoid the L.E seam. On the Super-D this will work great, as it is a constant chord wing. This allows the covering to "creep" as it is heated and tightened, and equalizes the tension. Of course, I finish with nitrate dope, which will bleed through and attach the covering everywhere it touches wood afterward. Polyurethane will do the same thing. Only difference is that poly doesn't continue to shrink over its lifetime, but as long as your material is initially tight, that shouldn't matter much.