First off, use a putty knife or five in one tool to scrape away any loose or flaking paint. Then wipe the area clean.
For gaps like these you need to caulk to cover the joint before you paint. Make sure that you choose a paintable caulk otherwise you're going to find yourself with a big mess.
Once the joint has been filled, smooth the bead with a credit card.
Next, fill in any nail holes with spackle. You need to let it dry, sand and then reapply if the spackle shrunk.
If you've disturbed the paint around the casing you'll need to spackle that as well. You want to make sure that the wall is even with the surrounding paint.
Sand the area and once it's dry and then repaint the walls and the casings! You'll have a great new surround to go with your beautiful door. I'm Mrs. Fixit and it's just that simple!
