If you are talking about the light ridges, usually about 1/8" to 1/4" apart, they are usually caused by improper cleaning of a cork grip and are somewhat difficult to get rid of. what I have found does work is to get a sheet of 400 grit snpaper, or higher, and then using a piece of wood that is thin but about 2" wide, use it as a sanding block and very lightly sand until the ridges are gone. the grip will be noticably smaller when you have finished though.

When a grip gets dirty and needs cleaning, regular warm soapy water will do the trick, and alcohol, used with a soft cottn cloth will clean them very quickly as well.