I have used probond to sheet foam on many airplanes, including a 123" B-17. The glue foams and expands into the coam core. It is the strongest bond I have seen. It is not light, and if too much is used, it will foam out onto the edges and through any cracks or gaps in the sheeting. I have also sheeted wings without joining the balsa sheets beforehand. I was trying to get rid of the unsandable glue line between the balsa sheets. It works, as long as the minnimum polyU glue is used. I apply a small amount of glue to the backside of the balsa skin, use an old credit card to spread and then scrape all the glue off that I can. The residual will be the glue that hides in the balsa grain. That is plenty enough to make an extraordinary bond. If your air is dry, spritz a very fine mist of water on the sheet, then apply. put back in the form and weight. Best bond there is.