Food for thought. Acetic acid, the active ingredient in vinegar, will form iron acetate on contact with steel. I haven't found any good info on the effectiveness of iron acetate as a passivation layer, nor on its effect on coating adhesion. Unless someone can come up with that kind of information, I'd avoid vinegar as a pretreatment. In contrast, the properties of iron phosphate conversion treatments and zinc phosphate passivation layers are well known, and actually compatible with epoxy. It's just that they are so likely to be misused, or to be unintentionally left in hard to reach areas, their use is not recommended.