Learned something new here ... cool info. I think that the upper part of the cowl and the rocker panels are galvanized. I imagine that dipping might attack that too. Or would this be the coating that you are talking about here?
Sorry ... didn't think I was going off-topic here ... as in that dipping the main shell would remove the aluminum coating and galvanizing leaving these areas unprotected ... so ... another reason not to dip the main shell and unibody.