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I use a magic eraser on mine and it work really well. I don't use a lot of pressure on it.
 
Cleaning the ipad smart case is fairly simple if you use the correct products. I like to use a slightly damp micro fiber cloth. This takes any dust and light dirt off straight away and restores the look.

You can also use a soft brush as well. For more stubborn dirt or grease that doesn't want to come off you will need to use more abrasive techniques. Use an alcohol based cleaner like alcohol wipes or isopropyl wipes. This works wonders just like it removes greasy fingerprints from your screen.

I wrote a lot on my how to clean ipad smart cover webpage http://mrproductreview.com/how%20to%20clean%20ipad%20smart%20case.html
 
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