hi there,
where in the country are you??
water run off from petrol stations runs through the site interceptors.
my personal preference for this work would be as litte water as possible, even with covering up the pumps you are going to blast all the grime over the light boxes, shop fascia the whole lot.
ideally a scissor lift, a degreaser, water and a pressure washer to rinse.
you would be wanting to put two people in this job, overnight working may be an issue if its in a residential area, sites have strict planing consent on operational hours, so be careful there.
we would be more than happy to talk to you about this if you wanted to,
martin.sales@btinternet.comregards
martin