Avoid primers. primers are, for the most part, permeable. Water will leach its way through to the base metal, which also includes just the humidity in the air.
Epoxy primers are a better choice, but still not ideal. Primers are, by design, meant to grab the base metal and provide a better surface for the top coat to adhere to. This means they all have a level of permeability to them. Its the top/clear coats that are meant to seal weather/oxygen away from the steel.
If you have to let it sit over winter where the weather is going to get st it, grab some type of rust paint (rustoleum, tremclad, etc) and give the area a good coat.
Give it a week to harden up and then tarp the whole vehicle to try and make most of the rain/snow roll off rather than sit. The rust paint is designed to function as a top coat and will do a better job sealing out weather than any primer will.
You’ll have to strip it off again when you’re ready to get back to the body work, but it should halt or slow down any further degradation while its sitting.
If you intend to drive it over the winter months rust paint will help, but its won’t stop it. Winter and salty roads mean corrosion. Its unavoidable, even on a brand new car where every spot they can get to is e-coated. Best you can hope for in those conditions is to slow it down.
”rust never sleeps”
So true.