You can get 2K epoxy primer in a spray can. I've used Spray-Max brand with success, although I haven't left parts primered out in the weather. Parts primered with it and stored in my unheated trailer for a couple years didn't rust. Once you activate it, it's shelf life is 2-3 days, but can be extended a bit if stored in the refrigerator.
Alternatively, you can mix up a little epoxy primer and brush it on your patches. You'll be block sanding it anyway.
The Ospho is a rust treatment, not a primer, and you have to be careful that your primer is compatible with ospho, which is acidic and needs to be neutralized at a minimum before you primer.
