We would say this product is essential when purchasing any whetstone. Keeping a consistent angle while sharpening is essential and impossible to do on a concave stone or one with ruts in it. Use this product between every 1 - 3 knife sharpening's to keep your whetstones in good working order. We use ours between every knife.

Remember that compared to the diamond plates we carry that this is a rough grit stone and will wear away over time with use. If you let your stones go too far between flattening sessions a very dished out stone will cause this stone to become uneven as well and you can't make a flat stone with two uneven stones, so flatten frequently with these!