How do I seal chalk painted furniture?

You can use wax, but that will need to be re-applied periodically. The other option is polycrylic.

I bought polyurethane to seal mine and it has worked very well to protect it.

Hello Cob! When you buy your chalk paint, most of them have a wax sealer to finish them. It's not necessary, but it looks great. The wax usually comes in several colors, to talk to your retailer and see if they have a sample board with the different shades.

I like the Polycrylic by Minwax. They make a clear satin or matte finish, depending on your preference.