Robert.... some guys use music wire taped to the building board with the balsa between the wire.  1/16 and 1/32 wire sanding the balsa to the 1/32 trailing edge. Using thicker wire for leading edge as needed.  Sanding with a sanding block. 

Others place sand paper down to hold the balsa and place at edge of the building board and sand with block to desired thickness at trailing edge.

Some sand the built wing panel by placing 2" masking tape infront of trailing edge, span wise from root to tip and using a long sanding block sand trailing edge.  Tape helps to keep from nicking the ribs and protect the ribs as the trailing edge is sanded. 

As has been stated trailing edge is tapered before construction is started.  Lots of choices, whatever works for you!

Tom

I taper TE's before building wing panels  No troubles ....

Using a long strip of double stick tape to hold a wing down while tapering the trailing edge. 

Results:  2 breaks in the T.E. while unsticking it and a ragged edge on the T.E. as I thinned it too much.

2 small pads might have been a better idea!

Weighing my options among rebuilding the whole panel or just grafting on a new piece of wood.  Not a stupendous amount of work in either case.