Kirk Hammett said he used 3 wah pedals in a song from their last album. I wish he would tone that shit down. How many wah solos has he done? 
too many
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3 wah pedals seems excessive, unless one craps out regularly. At that point why would you keep it in the chain?
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The only reason you would use 3 wah pedals is if you are making some weird-ass noise/sound effect, and given the nature of Metallica's music I doubt that is the case. Unless he means he played three solos in this particular song, and used a different wah pedal on each one.

To answer your question, pretty much all of Kirk's solos feature wah.