how much weight does sealing add to a ping pong racket?
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A better question is should I seal my racket or bat?
Sealing a blade adds about 2 grams to a table tennis paddle on average but it can add up to about 10 grams depending how thick you do it and how much of your raquet you cover - include sides and handle.
Some people do the handle to stop sweat getting in but it then makes your hand sweat more and the bat gets more slippery.
If you bought a quality blade then they are usually don't require any sealing. But if it is some cheap blade you will notice how porous it is and they must be sealed.