I plan to srape the paint off, sand smooth, fill the gaps and rough
spots, prime and re-paint.

Q: What is the best exterior wood filler for this use?


This isn't a wood filler but, think about a urethane glue for the
cracked boards. Mist the crack with water then squeeze the glue
bottle tip into the crack area. Use a wood shaving to push as much
glue in as you can. It will foam up and spread. Sands very easy, is
mostly waterproof and holds paint/stain well. Best of all, it won't
crack and fall out.

pete