Terry King swears that platinum over gum is superior to gum over platinum. I've tried it both ways, and I prefer the platinum layer first, if for no other reason that it is easier to register the other coats, and in my opinion, looks a lot better. The gum steps have the effect of filling in a lot of the porosity in the paper, so it also makes the platinum sensitizer a little difficult to soak in. Everybody I know except for Terry does it platinum first, then the gum layers. There are no rules, of course.