Commentary for Chagigah 53:4

וחכמים אומרים מפני שהן מצופין: אדרבה כיון דמצופין נינהו מיטמאו אימא וחכמים מטמאין מפני שהן מצופין

(Mishnah:) And the sages say, "Because they are coated [they do not receive impurity." (Gemara:) The opposite! Since they are coated, they [being metal] can receive impurity! I might say [that the Mishnah means], "And the sages [declare them] impure, because the are coated."

Rashi on Chagigah