Painting scabbards?

[Dan D'Silva]

Hello yet again.

I'm looking into having some new early Achaemenid akinakes scabbards (by a professional this time; I don't have access to a wood shop anymore).  I'd like to paint them again, but this time I'd like to use a historically-plausible paint, like casein.  So I need to come up with a combination of facing (or not) and paint that can be made water-resistant enough to not bleed in the rain.

The only facing I'm aware of that was used on akinakes scabbards other than embossed gold sheet is leather (the Issyk Golden Man burial).  I understand that leather during the Achaemenid period was sueded and something like buckskin or chamois, and my experiment with hide glue paint on suede showed that it dissolved very quickly when wet even if I added aluminum sulfate for water resistance.

What I'd like to try is linen, since I understand glued linen is easy to paint with casein, casein itself is somewhat water-resistant, and it can have a wax top coat applied, but AFAIK it would highly speculative to use linen for an Achaemenid scabbard.  The other, more parsimonious answer would be to leave off the facing entirely, though I've also heard it suggested that a scabbard with no facing or other reinforcement to keep the front and back halves together is at risk of splitting along the seams as it's jostled with the sword inside.

So I'm looking for evidence and opinions:
-  Were other cultures in West or western Central Asia, the Aegean, or Egypt using linen-covered scabbards in the early 5th century BCE?
-  Absent evidence for Persia specifically, is a linen-covered scabbard too speculative for your personal standards?
-  Can I make milk paint over suede work?  Perhaps by soaking the suede in glue to ensure a rigid surface? 
-  Is a scabbard with no facing at all likely to be too weak for use?