This is just one reader's opinion, but . . .
I've read pros and cons for both hourly and area (sq. ft.) pricing. One thing you might want to consider, with either system, would be a minimum "call-out" charge for any job. A relatively small "tag" on a visible part of someone's business might not amount to much in overall area, or the time needed to remove it. They want it
gone, as soon as possible, and you're the guy who can do that, right?
Even with minimal materials expended to remove a small area, you'll still have an "investment" in it. (Even before you get any calls about graffiti-removal jobs.) The time and fuel to get to the job, chemicals or blast media used to remove the mark(s), etc., will add up, plus you need to have the stuff ready to go when the phone rings. Can you recoup your costs (and pay yourself for your time put in) with the charge for an hour's work, or maybe ten square feet?
A property owner will probably be happy to pay a flat fee to get someone who will come right over and properly remove a mess. For larger jobs requiring more time/fuel/etc. from you, then you could add in your hourly or area rate. For smaller jobs, just charging your flat rate could make some buck$ for you.
I know that's kind of an evasive answer (something I'm good at
), but maybe it'll get a good discussion going about which system is better, per hour or per square foot.
BTW, I think you said you put in eight hours "de-graffing" a trailer. You put in a lot of time and work on that one. Did the customer specify no chems, or do you not use them for other reasons? I think chemicals would have cut the time expended on the job, but you still had a lot of area to cover.
Let's see who chimes in about which pricing system is better, and why. "Inquiring minds want to know."