I agree, if it was me I think I would try to sand off most of the old Hysol on the wood. However, I wouldn't stress too much about it because the bond of ribs to spar is not a critical structural joint. The ribs only serve to space the spars one from another and to provide an airfoil shape for the skin (fabric) to adhere to. The Hysol is only there to keep the ribs from sliding around and getting out of position. Your critical structure is the spars, diagonal cross braces, and the lift struts.