Jim,
Exposure of steel to the elements is a function of porosity of the surface coating/s.
Typical rattle can primers are an adhesion promoter designed for paint to stick to. They are like sealing steel with a layer or 2, or 3, or 8 of window screen. It dries quickly because there are many open paths to the atmosphere through the film.
Follow on top of that with rattle can paint and you really haven't gained much. Most aerosol paints are low in solids and hence hard to get any meaningful film build which would be what would be a must for gaining any degree on impermeability. I'm guessing the "paint" in those aerosols could be equated to window screen also, a finer mesh perhaps, but still porous. Anytime a product dries quickly, it's because there isn't much in that film slowing down the migration of solvents to the atmosphere. Hope this helps
Bruce Palmer
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