Use a real thin brush taped on a broom stick.
Unreasonable yes. Did you spray or pad? Was the deck sealed before you got the job? Were the cracks done? If not explain thats un nessasary labor for what you charged and charge for the extra service. Maybe she is just complaining about a few boards?
Big cracks? if so you should have knowen, small cracks well you should try and get out it is BS. 800 sq ft? It could of been worse.
If you used oil based sealer you are going to have to watch out or your going to end up with sticky spotting as well as shine and lapping.
If you use a spray like Dave said you could cut a strip in some card board tapes the edge lay it on the crack line hit it with the sprayer and wipe up any over spray right away.
Why do I like doing Deck Refinishing? I like taking somthing that looks like crap and making it look like new. Why? Good question! I hate painting! Deck Refinishers don't paint!