Painting old cedar plank siding?
 

I hate to be the bearer of bad news, but, you should scrape the whole
thing down. You will ensure that any loose spots that are not apparent
to the eye are taken care of. Don't look to get all the paint off,
it's not necessary, the loose stuff will come off pretty easily.

Get a good primer, there are lots of good ones out there, and a good
quality exterior paint and you won't have to revisit this project for
years. It's the prep work that counts the most.

If you've never used a sprayer for this type of project before, I
would use a brush. I've mostly found that the prep work and clean up
after a sprayer takes as long as if you'd used a brush anyway.