Self repairing armor isn’t prevalent in any legends in the medieval setting that I’m aware of, however it is a mythic concept which an Ars Magica magus might choose. The Magi of Hermes source book introduced the spell level 15 guidelines (p.39 and p.139).
An enchantment effect placed within a suit of armor can be written up as:
The Eternal Suit (CrTeHeAn 29) This suit of full mail is enhanced to always appear new and unblemished.
Sustained Regeneration of Armor
Creo Terram Herbam Animal 25, R: Personal, D: Sun, T: Individual
The armor enchanted repairs all minor damage, scars, or nicks, immediately returning the armor to an “as new” condition after being damaged.
This spell will also protect against spell designed to harm the armor, although it cannot protect against total physical destruction or extreme physical damage which would render the armor immediately useless.
(Base 15, +2 Sun) (+1 two uses per day, +3 environmental trigger: sunrise/sunset)
This effect will keep an item in pristine condition, however it will also make the object subject to magic resistance. This effect is also probably more desirable in a device which contains other enhancements, rather than a lesser device with only this effect.
There is also the weakness that if the spell was ever removed or suspended all the repaired damage would return at once, likely wrecking the item. Pragmatically a spell which makes items inherently stronger is better (like strength of adamantine), although as detailed in Magi of Hermes p.139 a ritual spell could permanently repair and item, very expensively.
A similar effect using Range: Touch and Target: Group could be used to target multiple items worn by a warrior. This is obviously better for applying the effect to a changing set of equipment, although that flexibility increases the final level dramatically.
A design note of difference is to also alter the effect from a permanent effect to an activated power. I suggest the user temporarily discards any weapons before activating so that they will not be resisted by MR.
A further consideration is the warping the spell inflicts on the items, which maybe of concern over many years.
The Eternal Tools of War (CrTeHeAn 46)
Sustained Regeneration of Arms and Armor
Creo Terram Herbam Animal 45, R: Touch, D: Sun, T: Group
All the arms, armor, clothing and equipment worn by the user will repair themselves against damage, nicks, cuts and scars, immediately returning the items to as new condition after being damaged.
This spell will also protect against spell designed to harm the wearers equipment, although it cannot protect against total physical destruction or extreme physical damage which would render an item immediately useless. This effect can target up to 100 separate items worn by the user.
(Base 15, +1 Touch, +2 Sun, +2 Group, +1 group size increase) (+1 two uses per day)
These spells are part of the free new spells compendium for Ars Magica.