Hello,

I did a few experiments earlier this month with painting soft suede using hide glue as a sizing.
gearing up
preliminary results
the rain test
Long story short, the paint flexes and doesn't seem like it would flake off, but it washes out very easily, even with aluminum sulfate added to try to waterproof it.

So, what I'd like to know is whether this is a reasonable method as-is for outdoor use, as I'd originally looked into it as one way of achieving the crenellated decorations on Classical gorytoi; whether I'm missing something, or should just abandon this line of thought and start over.

You could try a coat of linseed oil, cure well and it would provide some protection. Some other period appropriate resin might work to.

I have used that shields painted with casein and chalk. The drawback is that the brilliant white gets tinted slightly yellow.
Lead white would be nice, but is banned for a reason I guess..