This happened to my husband and I too. We had to buy the drywall dope and fill in the holes, chunks, etc. Then sand it. You may have to do a few layers of dope then sand, dope then sand, (there is no way around it being a horrific job) but be sure and don't glob it on b/c then you have to just sand all the excess off and that is hard work. Once you have patched all the chunks, holes and sanded it smooth take a dry rag and wipe it down to remove lingering drywall powder and you are ready to paint.