Glue Soak is the second upgrade for the Glue Gunner in Bloons TD 4 and the first upgrade on the 1st path on Bloons TD 5. When a Bloon is glued by Glue Gunner with this upgrade, the glue soaks through the bloon layers gluing all of its descendants. This does not affect M.O.A.B. Class Bloons as Glue Gunners don't attack them. Without this upgrade, popping a glued bloon will make the glue disappear. Gluing a Ceramic Bloon would be pointless without this upgrade, because Ceramic Bloons cannot be slowed down by glue, except in the Mobile/Steam versions.