As Chad suggested, we know that gesso/ plaster based approaches were used in at least some instances. An example; http://www.metmuseum.org/toah/hd/rarm/ho_25.26.1.htm

A primer on how to make and use gesso; http://www.redwulf.info/quill_instructions.htm

I still favor RustOleum for functional durability. For those leaning towards historical authenticity, I suspect the above is probably preferred.