You can work on Fiberreed like you do it on a cane reed. Therefore you need sandpaper grain 500–1000 and for the rougher work grain 300–400. To make sure that you do not breath grinding swarf you should use abrasive paper for wet sanding and wet the reeds before you sand them or wear a mask.
If your Fiberreed doesn’t response well or seems to be too hard you have to make it thinner. Normally it is sufficient to work on the tip of the reed. The sound will get a bit brighter thereby. Take a piece of sanding paper and sand the tip of the reed on the first third of the cut carefully. After each sanding play your Fiberreed to find out what has changed. You continue this process until you get the desired result.
If the low register is hard to play you have to work on your Fiberreed with rougher sandpaper at the rear part of the cut. This is the last third of the cut where the reed achieves the full thickness and changes over into the shaft. It is necessary to remove more material to achieve noticeable result. It is essential here as well to play your Fiberreed after each sanding.