To cover or close something with plaster or some other pastelike mixture. A noun or pronoun can be used between "plaster" and "up." We'll have to plaster all these marks and holes up in the wall before we move, or they might not give us our security deposit back. They tried to plaster up the cracks in the ceiling with paint, but it obviously did nothing to fix the problem.
plaster something up
to close something up with plaster; to cover over holes or cracks in a wall with plaster. He plastered the cracks up and then painted over them. You have to plaster up the cracks.