These are great comments, and very quick! Thanks for the help. I think will try wetting the balsa and plywood and the paper barrier to see if that works. Perhaps I'll revisit the soldering iron.

The jig is a good idea, but I'm going to rule that out for now because I don't really have a practical way of creating a jig exactly to the dimensions I need. The wings are not in the same vertical or horizontal plane, and my ability to measure things in three dimensions is rudimentary at best.

The heat sink is a good idea too, but not very practical in this application because of the design. The wing attachment points consist of steel landing gear straps that are soldered to the interplane struts as they sit in position on the wing. If i attached a heat sink between the strut and the wing, the strap would be out of place and could not easily be soldered in the right place.

The instructions do suggest that you flip the plane over before trying to solder the joints on the top wing. I suppose this will help somewhat with controlling the heat -- because heat rises, less heat would flow down from the irorn or the torch to the structure when the wing is below the solder joint.