How to edit knocks in piano recordings.

Recording pianos isn't the easiest thing to do well - as you've discovered. Especially if they are upright pianos; everything is pretty much in the same place, and all of it gets recorded - the performer, the music rustling, fingers hitting the keys, the sound of the hammers - all of this as well as the sound of the strings. Unfortunately it's very difficult to get rid of one of these without affecting all of the rest, and with relatively quiet percussive sounds (your knocks) it's nearly impossible without altering the attack sound of the piano. And that stops it sounding like a piano...

If this is an upright piano, then the most success I've ever had is to open the lid and suspend a couple of directional mics over the strings, pointing straight in. Generally they need to be nearer the top strings - you'll pick up the bass ones regardless. Get them reasonably close - the louder the wanted sound is, the less you'll hear the unwanted sound. And if you can arrange some sort of screening between the open lid and the performer area (hanging a blanket can help), so much the better. I've also heard that using contact mics can get over most of these problems, but good ones are expensive, and I haven't tried that approach, so I can't really say how effective they would be in practice.

If it's a grand piano the approach is different, and generally finger noise isn't a problem because you can get the mics further away from the playing area.