Looks good. If you are doing more of the same, stop at a real, local lumber yard and ask if they stock pre-primed, finger jointed board stock. You could rip a 1x10, or 1x12 in half, and end up with inexpensive base material that is far superior to MDF, and inexpensive. In my area it is stocked in 16' lengths, so you would also avoid a lot of butt joints. Another trick is to use vinyl patching spackle for the nail holes. Install with a finger, wipe with a damp rag, and you're done. I'm not sure if you are caulking the nail holes, but it can be a PITA, compared to using spackle.