Rent a pressure washer and blast it with a rotating nozzle (usually called a "turbo" nozzle) or if you get really lucky they will rent a thing that looks like a lawn mower or floor polisher and the washer hooks up to nozzles that spin underneath and REALLY blast the surface (they are what professional sealcoaters that need washing a lot use), you can just go over the drive like mowing a lawn real slow then rinse with a garden hose and those or a turbo nozzle will blast off anything. The sealer will stick better then your "regular" jobs.
If all they have is regular nozzles get as small as possible for the most pressure but it's a lot slower. The others like turbo nozzles have a rotating 0 degree nozzle so they blast a 6-8" circle as fast as you can go over it-much faster than a regular washer.
Or you might sub it out to a washing contractor in your area-they probably already have all that and you can add it to the bill by giving the homeowner the choice-pay the washing costs and you guarantee the sealer-no washing-no guarantee...that usually sells them on washing easily.
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