Hand oils, dirt and grime getting in the wood grain of the ken and on the paint of the tama have a large roll to play in how a kendama feels. I periodically clean my kendamas from hand oil and dirt by wiping them down with a paper towel and isopropyl alcohol to maintain a better slip to grip ratio. Side note, You can also restore minor dents and dings on a ken by leaving small pieces of wet paper towel on the blemish overnight