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I glued slabs of cork 10 cm thick on a ceiling. Cork is not smooth. Can I smooth it to the smoother and paint over it?

It's probably about 10 mm cork tiles, not 10 cm... It's still possible toto coat cork slabs to make them smooth, but the work will be consistent, for a result that may be relatively disappointing. In addition, the joints between slabs will crack with time and the grid of the slabs will reappear.
If the goal is to enlighten the ceiling, as much paint the slabs with a matte acrylic paint. If the goal is to find a smooth ceiling, even to coat and sand, as well to deposit the slabs to work on the original ceiling.

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