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 there will be a battle with Bowser Jr. for the second time but he will only appear once the player collects the first three Royal Stickers and has visited Worlds 4-1, 5-1 or 6-1. After the player beats Bowser Jr. Worlds 4, 5 and 6 will be available. It is required to beat the Big Cheep Cheep in Surfshine Harbor to unlock this area.
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