I try to use materials that will stand up. Unfortunately this sword took heavy damage in our last move. The next time I have to make a sword of this size (Guts' Dragonslayer, I'm talking to you) I'm thinking of coating the foam in a layer or two of acrylic paint and then using Bondo. I need to test how well the acrylic will seal on a scrap piece of foam though before I attempt this.
If you use acrylic drywall primer it does a really good job of protecting the foam. I prefer Dulux because it can be sanded and if you finish the sanding with steel wool you can polish it to a high gloss finish. Some swords I made have had as many as 15 coats of primer on them.