Using Corn Syrup for Shine

I've applied corn syrup to small transfers to give them shine (never to an entire cookie). But it remains tacky for a very long time, and, because it's sugar, it's going to absorb moisture whenever conditions get humid. So if it seems to have dried, if it gets humid, it will get tacky again.

I agree, I’ve had the best success using corn syrup with a light hand in small areas. I brushed some onto the patty of a hamburger cookie once, and I’ve kept it in a bag for years with no issues. (I do live in a pretty dry climate though, and the cookie lives in a box with desiccant packets as well.) My stained glass cookie, on the other hand, has a thicker layer of corn syrup and is still tacky a over a year later.