Open cell foam is full of cells that aren’t completely encapsulated. In other words, the cells are deliberatly left open. This makes the foam a softer, more flexable material.

Closed cell foam is made up of cells that are, as the name suggests, completely closed. The cells are pressed together, so air and mositure are unable to get inside the foam. Because of this, closed cell foam is much more rigid and stable than open cell foam.

Yes, it comes in cans. The open cell sucks moisture in. which will lead to more rust. I have seen mold on open cell foam.