Like Ron says, the rust will most likely will come off with the paint.
Test an area and make sure you discuss the expected results with your customer. You should almost be able to tell the results by spraying water from a garden hose and rubbing your hand across it. Again as Ron says, sell it as paint prep and if it comes out better, than nothing is lost.
Phoshoric acid based cleaners also work well on rust streaks. Causes no damage to paint in good condition (rubbing and pressure may).
Not sure what is in the one Ron recommends but be careful, not all toilet cleaners are the same, some contain bleach (Sodium Hypochlorite), some contain Sodium Hydroxide (which will strip paint above 1% level) and still there are a few others including some which contain as much as 20% Hydrogen Chloride which should not strip paint in good condition but will remove rust (Hydrogen Chloride / Hydrochloric acid / Muriatic acid - the names are synonomous but whatever it is called, it should not be mixed with bleach as it will give off dangerous gases).