Walberg - I'm probably opening another can of worms here, but I wouldn't dry mount to any kind of foam board. I have seen the board collapse, or compress in areas, which can be seen viewing the print. I've also seen foam core do this in sections, if the heat in the dry mounting process is too high (or long, or pressure too much).
Why do you mount to foam board, anyway, just curious?
You can buy self-adhesive foam board, so why would you want to use the archaic process of dry mounting?