What I have been doing is this:
I can only get the reo Flex 30 and the Reo Flex 50 in Mexico City. I have been buying the 30. The 30 is too runny for brushing on, so I thicken it a little with Cab-O-Sil for the first coat, and even more for the next coats.
I plan to try the 50 for the thicker coats. Using only the 30 works, but I would like the later coats to be more like a paste.
Also Smooth-On makes a polyurethane called "Brush-On" which I used once. It is thick and eliminates the sagging off problem, but to use it as a first and second coat on oil based clay is a little dangerous to the clay as I needed to press on the brush to fill in the details. Maybe two coats of thickened 30 and then "Brush-On" for the rest of the build up would be a good way to go.
Smooth-On has a distributor in Oz:
Rowe Trading
4 Holder Ave.
Richmond
South Australia 5033
Tel: 61-8-8234-0000
Fax: 61-8-8234-2092
fwoods@rowetrading.com.au sales@rowetrading.com.au
http://www.rowetrading.com.au/
Richard