DON'T let the dealer FCUK it up. Please, please, please, do it yourself. I assume by "flash rust" you mean industrial fallout, e.g. little bits of iron that have bonded to the paintwork and started rusting
You could clay the car - but the easiest option would be some Iron X
http://www.cleanyourcar.co.uk/engine-and-exterior/carpro-iron-x-500ml/prod_726.html(also available at other places)
Clean your car, dry it off, spray that all over - I leave it 5-10 mins, then agitate it with a microfibre - and wash it off (smells awful, make sure you wash it out of all bodywork gaps, door shuts etc)
Problem resolved. Then get a good, durable wax on there (I suggest Collinite 476) to help prevent this happening in future
Again - don't let a clueless dealer try to remove it, god knows what they would do.............