Someone on knifeforums said that corrugated cardboard is what Mike Stewart, of Bark River Knife & Tool, uses to keep his personal knives sharp... but BRK&T knives are a bit unusual in being full-convex ground, like Fallkniven knives, and thus can be stropped flat against whatever... other folks, though, report no success with the method. I owned a couple of BRK&T knives briefly, but I didn't hang on to them long enough to need to sharpen them.

Cardboard carries lots of fine abrasive grit, it's routinely used to dull knives, or get some feel as to how long they'll hold an edge, so it should work for sharpening with different angles. Note that Trace Rinaldi feels that because of it's 45-degree micro-shearing (as opposed to steel's 90-degree), LiquidMetal (LM1) actually can actually get sharper from cutting cardboard, not duller!