Believe me, Polyurethane glue can be used and kept light. I made some balsa ply by spreading Elmers Ultimate with a PUTTY KNIFE! I used just enough to dampen the surface of the wood, and scraped the excess off with the straitedged blade, sprayed water directly onto the glued surface and set my crossgrain stock in place and weighed it down. Moisture is the key to maximum expansion of PU glue, and the more it expands, the less you need and the better penetration you get. I've pretty much quit using epoxy on any high-stress joint I can clamp.