If the rust isn't taken care of and is just painted over, then the cancer will just spread. Primer will not protect metal either because it's porous. I see a lot of primered only cars and cringe at how much rust is under it.
a primer sealer will not allow moisture in and can be used by itself temporarily but you still need to take care of the rust problem before priming. there are certain products you can use to "paint" directly over the rust such as POR-15 or Eastwood's Corroless but you can't use these on body panels or at least it isn't recommended for them anyway. it's ok to use them on the insides of body panels but not for exterior parts that will be painted.