The trick to extira, is finishing both sides of the panel. if you only route/sandblast one-side, the other factory-finished side will cause the panel to potato-chip over time.
I'm sure your client wants to use Extira since it holds up well against moisture without special finishing, but I'm curious why he wants sandblasted vs. cnc routed, as Extira has no woodgrain pattern to expose. Regardless, with the right combination of Straight/V-engrave/Roundover bits you should be able to CNC pretty much any effect he's lookin' for.
attached pics of my recent Extira endeaver. I wasn't a fan of previous jobs where we were of having to stop the "woodgrain" pattern in a perimeter around the lettering, to account for the V-bit profile not cutting into them, so I mounted square-cut letters on top of the field of woodgrain instead.