Panel beaters use phosphoric acid on the steel prior to painting.
This protection can last for months, but painting is best done as soon as the acid has been rinsed off and the steel is good and dry.
Phos acid is one of the active ingredients in coca cola, so its not exactly hazardous to life and limb.
Best kept away from eyes and clothes though...
Don't dilute down too far though, I find its best used really concentrated, and then sandpaper off for a smooth rust free finish.
There is no substitute for this product ?
It should leave the steel clean and shiny and metal-like, none of this black coloured stuff that weaker varieties use.
Dulux used to supply it as Deoxidine, but many suppliers of it these days.
Don't pay too much, its only a powder mixed into water.