If you plan on keepin the car then Priming over ANY rust at all wont do anything and it will come back.
Either cut/grind every last spec of rust off or use a rust converter product.
POR-15 has a good one called METAL_READY It is a converter and Zinc-Phosphate primer in one. The Zinc prevents further rusting. POR-15 paint must be used to seal the area.
There is also Rust Converter stuff at Autozone but I was not thrilled with its performance.
POR-15 paint absorbs moisture to cure and sucks it out of the affected area helping to disable the rust.
KS Auto on River Rd in Tonawanda has POR-15
Rust is like a cancer and it WILL continue to spread if its not completely removed or treated just right. If you are not keeping the car then whatever but in this climate you will probably see it back in 2 yrs.
If you dont use any of the products mentioned then I recommend you heat up the area with a propane torch to drive off moisture JUST before priming/sealing. There is a lot of moisture in the air (and embedded in metal pores) now. You need to heat it very lightly. You will see the color shade change a bit as the area warms up and a line will form. There will be a dry side and a side that is sweating moisture. Move the torch behind the line slowly till the dry are spreads as far as you need it.
Obviously don’t heat the panel more then needed. Seal both sides of the panel (very important)