How to clean the bottom side of the inverted rubber?

[confused_zeus]

Looks like stains to me. BTW, is that your primary blade? How many times do you play in a week?

 

[Maximilian2016]

Looks like stains to me. BTW, is that your primary blade? How many times do you play in a week?

Yes, that was my primary blade and I have played with it for about one year, 3 or 4 times per week, 2 hours per session, with people and tennis table robot.Recently, I have switched to another blade and this is why I am looking to clean that dust because I am going to keep it in the case from now on.