To add to Paul's answer (which is spot on IME)...

Properly applied the design in question should not have lots of structure in the way. If you look at models that are designed for single surface type airfoils the leading and trailing edges ARE the spars and only a few ribs break the bottom surface. Any spars used in this method should be flat and in contact with the covering. I've seen models that use rib and spar designs that are meant for two sided covering only being single covered and that, to me, IS a mistake. Even at the lower speeds you're still making a lot of drag compared to the thrust power often used on this sort of slow flying model. If you want to do a single cover job the structure should be modified to suit so that the structural elements are snuggled up close to and in contact with the covering.