Hmm, seems like I didn't explain myself very well.
I also cover the whole balsa core with double sided tape, it works well. Then I use solvent to joint the upper and lower surface along the trailing edge. Finally, I bond the wingtips, also with solvent
I'm considering substituting double sided tape to join the upper and lower surfaces at the wingtips. It would avoid getting too much solvent into the joint causing shrinkage, and it would be quick, so the surfaces don't get a chance to move around
I'm going to try that on my next wing
Incidentally, I always split the leading edge of the wing covering at the wingtips with a scalpel. It gives somewhere for the excess material in the lower surface somewhere to go.
Richard