Zach is apparently left handed and can do the salute just fine with his left hand, but has problems doing it with his right? I have seen the glue thing before, I have also seen an interview where he said that it was just for practice and while filming there was no glue involved, but when you look at the scene his hand does curl funnily.
In the end it's a funny story and I'm not all that invested in how much of it is true...