I use the brown box call chalk or rain chalk. Rain chalk won't gum up on you if you run into a little moisture. Apply the chalk only to the paddle and not the sides. Also for removing chalk build up just use a green scotch brite pad with very little pressure. This is what a custom box call maker told me to use and all of these work well with no damage to your calls.

Dr. Andy stole my comment. I use a chunk of my grandkids' sidewalk chalk. Works fine. Keep in in a small zip-lock bag to help keep the moisture out of it.

Chalk colors mean different grades/densities in the material. If the chalk is to dense it will load up and gum the material. The reason we use chalk on wooden calls to too draw out as much moisture from the wood to allow it to create a better friction. Most wooden calls will work without the chalk if it is sanded more often creating the sanding friction groves in the wooded paddle.

The water proof box calls can also be conditioned when the water proof cover on the underneath paddle creates smooth shiny marks across its lid and causes the lid to slide rather then grab. Use scotch brite to lightly rub the shiny spots to a dull finish without taking off the covering. For a better friction sound between the lid and sides sounding board or rail you can take a very heavy grit sand paper, like (80) and run it along ways up the paddle from the screw end to the handle only working it one way- not a back and forth sanding motion and only allowing the heavy scratches to go over the material to get a better friction. My opinion is they sound and work way better after some use...