Although almost all of you disagree i would use acid solder instead of rosin core. The acid has more strength and will grip better. Rosin is made for more of eletronic stuff. Roughen it up put some paste if you like and use 60% tin and 40% lead better yet get some with some silver in it. (Hard to find). I was having the same problem with my Kadet.

Papa

P.S. Make sure you solder of your plane on put some protection infront of your plane dont want a hot piece of metal falling through your wing or something.