From the Super Mario Wiki
The Decal Channel is a small area of water separating World 1 through to 3 and World 4 through to 6 in Paper Mario: Sticker Star. It is required to pass through it after beating World 3 and defeating the Big Cheep Cheep in Surfshine Harbor. After entering World 4-1, 5-1 or 6-1 and returning to the channel, Bowser Jr. ambushes Mario and fights him for the second time. After the battle, the rest of the levels are available.
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