A very easy tutorial for this Faux Rust Effect can be found at thicketworks.
This is my first attempt.......I started with a gesso undercoat on the paper/cardboard figure. Let this dry.
Next I mixed Prussian Blue & Burnt Sienna acrylic to make a black & covered the figure.
Tore pieces of paper napkin & stuck these on with heavy gel medium,
Next I placed Burnt Sienna acrylic randomly, sprinkled cinnamon over the wet paint & pressed it in with my fingers.
With a stiff dry brush I removed the excess cinnamon.
The big patch in this photo shows where I put gel medium on top of the Burnt Sienna & sprinkled cinnamon over it & again pressed it into the gel, this added more texture.
I like this effect, with some more experimenting this could become a favourite......many surfaces could become Rusty!
Thanks to Heather from thicketworks. A comprehensive tutorial can be found on her blog.