I called the good folks at Smooth-On today and talked about the trouble I'm having with getting a thin layer of urethane to work in my 1630 mold. The helpful chap I spoke to recommended their "Brush On" line, which makes a whole ton of sense and I'm sorry I did not think of it myself, or sooner.
Here's a link: Brush On Series
I'm going to order a sample kit to play around with, and see what kind of results I can get.
Thought I would throw a little terminology out at you too. If something is low viscosity, that means it's more like water. If it's high viscosity, that means it's more like syrup.
I was told that the brush on urethane can be thinned with isopropyl alcohol, by the way.