I've done this...it's hard work. It will take the average DIY'er a long time. I'd just patch the areas needing it. You'll get a better result if you pay someone to put up a finish coat. That's an acquired skill you'll likley never accomplish especially on a ceiling without a lot of practice. they make it look so easy...it isn't! the average person will have a really hard time working over their head unless they have a job that requires that and are used to it.