Responsa su Deuteronomio 6:9

I would say that, if so, he would disqualify most Torah scrolls, tefillin and mezuzot²¹³The Oral Torah has interpreted Deuteronomy 6:9 and 11:20 as actually placing scrolls with these sections on one’s doorposts. These are called mezuzot.! As in now routinely done in many cases, when the ink of those parchments is drying up because of age (or environment), one takes it to a scribe who goes over it with a quill to make it look ‘good as new’. This religious law is explicit in the Shulkhan Aruch, Orakh Khayim 32:26: “If the letters of the words were erased slightly, if their impression is still recognizable a little bit such that a child…, it's permitted to pass over it with a quill to improve the writing and renew it, and this is not considered [written] out of order.”