Guys, guys, guys, if it's rusting you're after why simulate it when you can use the real thing?
For years now, I've been using my own method of applying REAL RUST onto enamel painted hulls without the need to over-coat with varnish.
Years ago, under my old kitchen sink I found an old, brown and powdery steel wool pad and immediately knew what to do with it. I mixed a quantity of the rust powder with water (and later with a tiny bit of dish washing detergent) then brushed it directly onto the hull of my model. I let it air dry then lightly brushed away any excess. A couple of applications may be required to get the effect you're after. Purely by accident I discovered that it permanently stained enamel and didn't wash off during sailing, i.e. no need to over-coat. Model Boats magazine published my method last year but you can have a look at it in my "Hints...." page at www.pier72.com
Trust me, it works!!!