Oh, okay, sounds like drywall to me...I think the best approach would be to get a large drywall trowel (that big rectangular bad boy) and use it to cover that wall with a thin layer of Topping Compound...let it dry, then sand it out.
Then, before painting, there's this stuff you roll over the fresh "mud" as a sort of primer...can't remember the name, but your paint store guy will know.
Hope that helps!
