Should one Repair a Torah Scroll or send it to Genizah (burial)?

One would think that, as in other areas of life, the old adage “Yashan mipnei chadash totziu” (the old should make way for the new) would rule. Thus, when a Torah scroll begins to age, it is preferable to bury it rather than send it to be repaired. Halacha, however, does not agree with this premise. When it is a Torah scroll that one is referring to, and considering that this is a holy artifact we are talking about, we do not apply the same rules that apply elsewhere.

What causes a Torah Scroll to Age?

Torah Scrolls often age as a result of the act of gelilah (roll-up). During gelilah it is forbidden to touch the parchment with one’s hands, and it is customary, where necessary to handle the scroll using a scarf or a tallis (Prayer shawl). As a result of the difficulty of rolling up the scroll while keeping the parchment tightly stretched and pressed together, it can happen that the chazzan will sometimes have to handle the scroll with two hands in order to create a window that is wide enough for the person reading from the Torah. Human touch means the transfer of perspiration and moistness to the vellum which causes cumulative damage.

Another cause of damage to the script and the erasure of letters, are creases and bulge that form in the parchment. These usually occur once the halfway point in the scroll is passed. At this point the roll-up becomes awkward causing bulges and creases to form in the parchment, which can lead to cracks in the ink and breaks in the letters.
Common Problems that Need Repairing in Torah Scrolls

Torah scrolls may become invalid when their ink begins to erode and disappear, or their letters are damaged. The question is what is the Halachic red line at which point a Torah scroll is no longer reparable and needs to be sent to genizah (halachic burial of holy artifacts)? What is considered a mistake in the scroll? And are mistakes accounted for by quantity or quality?

The source for answers to these questions is in Tractate Menachot (29B). There it is stated that a Torah scroll with three errors on each and every page needs to be fixed, while a Torah scroll with four errors on each page must be sent to genizah.
Is Every Error Reparable?

The Gemara distinguishes between two kinds of problems:

1) An error that has resulted from incomplete writing — this must be fixed by adding
2) Excessive writing (too many letters) — all mistakes of this kind may be fixed, without the missing letter, up to three mistakes on each page. For example if the word Bereishit is written without an Aleph, the missing letter must be added in.

limitation, without having to send the scroll to genizah. The errors are rectified by erasing the superfluous letters (scratching the ink off the parchment and rewriting the word).