I will say I do not know.  This is my next step in my build.  I can tell you what I plan...

I plan on epoxy coating the ends of the spacers prior to install.  The full length inwale, I will epoxy coat the (the whole thing), then apply thickened epoxy to spacers, then clamp up.  The spacer side will be sufficiently sanded for good bonding  We will see how it goes.  Varnish will be at the end after assembly.

Every step is an adventure and learning process.  :)   

   I'm sure there are many ways of doing it but I just finished my NED with the spacered inwhales and I assembled them just like the plans say.  I wanted to make sure that the bonding surfaces were perfect. I kicked around the idea of precoating and decided it would be too hard to keep the bonding surfaces smooth and clean enough to get nice tight bonding surfaces.  For me little drips and globs of epoxy get everwhere expecially on so many surfaces.  After it was all glued up I just took my time and used the Jen Manufacturing 3" foam brushes to apply the epoxy and the varnish. to the inside surfaces. I went around catching runs and drips until the epoxy kicked off which made for less sanding and cleanup later. The insides of the spacers arent glass smooth but they look just fine. I worked hard to get the most visiable top surface looking like glass and that distracts the eye from any imperfections down in the spaces. I wont lie it was really tedious and slow but well worth it in the end.

Pete

Thanks Pete!  Good advise.