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.)

Any white glue has more moisture to it then CA, and I would probably say your correct in that in this case using 2 different glues is what warped your stab.

When sheeting your structure, you did put glue on the structure only and then placed your sheeting on that....right? And actually it should be rather sparing on that too. A good fresh bottle of medium CA should allow sufficient time to align and place a sheet though, and with CA you can get somewhat generous(moreso then white glue), and that in itself gives you extra work time. For some reason though, I'm thinking if it warped you had glue on more then just structure, especially if it was flat overnight. Just ideas.

agree,don't think 1/16 on this plane will matter,also on covering you can stretch one side to re level the stab.,that amt..I would fly it and not worry.