I use Martin Senour (NAPA) or DuPont phosphate conversion coat, it goes on with a brush or sponge, then is buffed off with a damp rag. I think it puts an iron phosphate coating on the surface of the steel, which inhibits oxide formation and etches the surface to promote adhesion. If you ever carefully and fully strip a factory panel, you can see the same darker gray finish. I have had very good luck with this, but it can be tricky to get right. For instance, if you water wash instead of dry buffing, you get the yellow/brown oxide. The key is to keep excess water off (or any water really). I used to lay down primer over bare steel and if it got a chip or scratch it would start rusting and never stop until you sealed it up. Also, it was prone to chipping. All this being said, I prefer the self etching primer for convenience but still think you should conversion coat if you have time.
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