First you have to paint one side of the bookshelf. Then you have to wait for it to dry so you can turn it over. Then you have to paint the other side. Then you sand it down, turn it over, and sand some more. Then you paint one side of the bookshelf. Then you wait for it to dry and paint the other side. Then you check whether any more bits need painting, and they always do. Then you have to paint the shelves, which is much the same process only fiddlier. That’s how it is when you’re painting bookshelves.