If the boundary is still there when you have selected a drawing tool (ie the fill tool) then it is actually on the image itself. So either do as Jen said or you may have to rub it out by hand or use the magic wand.
roryt
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If the boundary is NOT a border or the edge of a content box, but is just a visible edge of the top layer, it is generated in the human eye. To make that go away, the pixel colors at the edges of the top layer must match the pixel colors of the bottom layer where it meets the top layer.
MidiMagic
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