Whenever I deal with a maintenance client with multiple cars I let them know that to maintain a vehicle *I* require the paint to be to a certain standard; not perfect of course, but kept well enough that the weekly/bi-weekly/monthly upkeep won't require an entire day. So for the first visit I normally wash & wax any/all cars and set aside a little time for paint correction on say, one of the cars... for the ongoing visits I continue to wash & wax the cars and correct one/two more each time until all are corrected enough that I can just show up, wash them, and be gone. For daily drivers generally they don't want perfection; but for the garage queens they do, so I tend to spend more time on those getting them just so.
For clients like that I just charge an hourly rate minus say 10% an hour, so basically they get a free hour of work each visit (until I am done with the correction, then they just save ~$10-$30 or so a visit).
