Stickier Glue is an upgrade for the Glue Gunner. It costs $100 on Easy, $120 on Medium, $130 on Hard and $145 on Impoppable. It makes glue last twice as long on bloons (12 seconds to 24 seconds in BTD5 (?) and 11 seconds to 24 seconds in BTD6). When combined with Corrosive Glue, it is highly effective because it gives time for the glue to dissolve the bloons more effectively.
In BTD6, Corrosive Glue without this upgrade can only pop a Rainbow down to Reds without additional pops. Purchasing Stickier Glue will allow an entire Rainbow to be popped completely, provided that there's enough space for the Rainbow Bloon to go through the track without leaking.