I'm a little late for the party and really liking this. It's about time a Porsche got the rust treatment, especially using the excellent 356 kit.
It looks like the rust technique you were using involves one of those using real iron pigment paint. The one I'm familiar with is Sophisticated Finishes. If I'm wrong, just ignore me. This works best if the body is placed in a plastic bag after applying the top coat that causes the rusting. I get very little reaction when I leave it out in the aid to dry. What you were experiencing looked similar.