When a whetstone is used to cut metal, it acts like sandpaper by removing small particles of metal (aka “swarf”) with each pass of the blade over the stone. Therefore, whetstones with more coarse grits cut faster than those with finer grits and, at the same time, soft whetstones cut faster than hard whetstones because each pass of the blade over both types of whetstones not only removes fine particles of metal from the blade, it also removes fine particles from the surface of the whetstone (aka “slurry”) which continuously exposes new cutting crystals. However, if the swarf is allowed to build up on the surface of the whetstone during sharpening, it will clog the stone and drastically diminish its effectiveness. Therefore, some whetstones require water to lubricate the stone and suspend the swarf whereas, other whetstones require oil to lubricate the stone and suspend the swarf.
I recommend keeping two stones in your kit. One with a medium grit (around 800 or so) to perform major sharpening jobs, and one with a fine grit (at least 2,000) to tune the edge to a razor-sharp finish. For real pros, a stone with an ultra-fine grit (8,000 and above) will leave a mirror-like finish on your blade, but most cooks won't notice the difference in terms of cutting ability.

Ease of use – Most people in search of a mechanical sharpener want one because they don’t want to be bothered with trying to achieve a perfect edge themselves using a stick or sharpening stone. They want predictable, first class results every time. In that case it’s important that the electric powered device is easy to use, achieves results quickly and with little effort and is designed with user safety in mind. Keep in mind too that it’s easy to apply too much pressure when using a mechanical sharpener and when that happens you’re likely to see unsatisfactory results. In addition there are subtle differences between mechanical devices designed for Asian-style knives and those designed for Western-style knives. This has to do with the sharpening angle discussed above. Don’t get an Asian sharpener if you don’t need precise control over your cuts.
first time buying a sharping stone. i bought this because i wanted to sharpen kitchen knives. you have to practice to get good at it, use a knife you don't care about because you might mess it up. there is some conflicting instructions online on how to use, water or no water. read as much as you can about using stones and practice, practice , practice. look up videos on youtube and internet. i had fun using it, BUY IF YOU WANT TO LEARN here is some stuff i found, they all sound like professionals http://video.about.com/culinaryarts/Sharpen-Knives-With-a-Whetston.htm?rd=1 http://culinaryarts.about.com/od/knivescutlery/ht/whetstone.htm this is a video that says don't use water http://video.about.com/culinaryarts/Sharpen-Knives-With-a-Whetston.htm?rd=1 this is a youtube video that i watched that uses water. *informative http://www.youtube.com/watch?v=KFIg9Cm-nJg
Our large selection of stones from many well-known manufacturers will allow connoisseurs to find the ideal stone for their needs. Because all manufacturers formulate their stones to emphasize a different mix of qualities, and because these qualities can vary widely between different stones, most woodworkers choose stones from several manufacturers to build up an optimal set of sharpening stones. Then again, once you get to know the characteristics of certain types of stone, you may find one supplier who will provide all the stones you need. Sometimes this can be an advantage. But there is no one size that fits all; each stone must fit your needs and work style.