What do they paint waterproof calls with?

[Owl]

So quick story, I was hunting some public land last Fall and passed an abandoned hunting shack that was falling apart. On the way back down from the hill I decided to stop for a sit inside the shack when something laying on the floor caught my eye. Nestled in the debris, under a rotting bench, up against some poison ivy was a box call!

I picked it up, blew the dust off, and gave it a run. To my suprise it sounded great! I now carry this call as it's the loudest one I own and can get a surprisingly wide range of tones out of it for a box depending on how much pressure I apply to the lid.

I was curious if anyone knew what they paint them with to make them waterproof and run this way? I like the sound plus not needing chalk.

[Yoder409]

Chalkboard paint

[Owl]

So, chalk and chalkboards make turkey talk! Thanks!