Once metal is exposed to the air it can start surface rusting within 1 hour after the paint is off. Most hobbyists use zinc chromate primer or whipe the surface does with a zinc solution like "Oxy-Solve until they can get to prime it. Don't rely on primer too long though because primer is pourus and will rust through after a few days depending on how the car is stored. You may want to put on a few coats of primer if you're not going to get to painting it for a while.

according to a photo that came with a 1955 packard we are restoring it was black but we also found traces of a red on the engine block when it was boiled at the machine shop