Editing Foam Background Guide (section)

=== Step 5: Shave off the surface of the foam ===
Once the expanding foam is done curing, and you're done attaching your hardscape, it's time to remove the shiny surface layer of the foam to add texture and improve the adhesiveness to silicone.

After putting on your protective eyewear, face covering, and gloves, use either a utility knife or drill with a wire brush drill bit to remove the shiny surface layer from all of the expanding foam. 

This is also a good time to carve texture into the foam, if there's a "look" you're going for.