Yeah, I admit rain was sort of a stretch for this discussion, but Zappa talked about a project being moved, his car moving and though implication loosing control of the project and storage for a few months. I figure that may include something other than garage cover and some rain. If I were going to spray top coat in a week or so with no moisture exposure, I wouldn't hesitate to use etch. More time or more moisture exposure, I wouldn't use etch without top cover. It really relies on the top coat to seal water out. For a project that is open ended (get back to it once moved and the dust settles) and it needs to be "closed up" for protection, I would use an epoxy primer. I like PPG's DPLF. My photo shows my conviction to PPG. (I like Dupont too, but the local shop isn't too friendly to diy) I won't trust anything less because this is a hell of a lot of work. Many say the SPI stuff sands pretty well, so that is why I suggested it. DPLF sands just fine when it's a week old. Three months from now, rollered DPLF will block sand flat, no problems. In fact it's necessary to sand epoxy and recoat it after seven days.

I have applied DPLF epoxy with a brush and roller where the gun can never reach. Where appearance doesn't matter (in my instance inside the cowl panel) or if you can sand it to continue the job I think it's the way to go over bare steel.

My no 1 choice in this instance would be to bring a gallon of PPG DPLF and enough activator to a "spray and pray" shop and pay them $300 to cover the car with two coats. Those guys paint a lot of cars and they could crank this one through with a nice job on the primer. These low budget shops spray lots of cars and the bucks is from skimping on the prep and products, which here is provided for them. That keeps the job moving, assuming this is truely bare steel and really ready for primer. If not ready for primer, I would look at Picklex. That might hold things a while. I doubt it would guard against rain though. Anything above 50% humidity seems to cause problems here, much less one rain or some outside storage. Even one night bare metal outside at 90 percent humidity had set me way back, lol.