Snoli Alu Oxide Stone (120*20*6mm)

Alu Oxide Stones are used to aid the ski/board edge sharpening process; if your file fails to bite use this stone to remove the case hardened area then back to the file. 

Check your sidewall isn't preventing the file from biting either. 

 120*20*6mm

(case hardening happens when you get sparks from metal hitting rocks and sparks fly.  These sparks carbonise the small area around the contact making the metal harder in this area.)