Mixing is working fine and the filter attachment allows a good mix of content.
What the strength of the seal may concern: I have tested several shakers and even the best (this one is definitely among the best) needed each patient during closure. The trick is this: First, you set the shutter entirely on straight, without pressing. Then one turns so long in "the wrong direction" (ie as if you wanted to open) until it clicks into place something down. Only then one turns to, without tilting, very slowly and loosely. Then you turn on until it is firmly closed.
Saw it firsthand on numerous occasions that a hasty about, impatient closure has led to the Shaker canted and the content runs out during Shaken. But this is a human error. So let there be no rush!
