That would be the perfect area to use the safestrustremover/evaporust and rig up a sump with a flower sprayer and a plastic sheet to drain back into a catch basin. This was the kind of surface rust that I cleaned up on the '72 Trans Am a few years back. Find a plastic Kiddy pool at ToysRus and get a cheap pool cover sump. Five gallons of the liquid will do just fine.
