When glueing up the 4*60 horizontal stab, I wicked CA between the exposed frame and the first balsa sheet. To be sure I had more positioning time for the second sheet, I used Weldwood white glue for the second sheet and pressed the stab flat overnight to dry. The result is that each end of the stab sag a bit more than 1/16".

Is it possible to somehow flatten the stab before I attached it?

Is a sagging stab inevitable or is it the result of using two different glues? (It appears as if one of the glues caused a contraction and/or one of them caused an expansion.)