You should be able to place a damaged armour piece into a crafting grid with one or more pieces of the material it consists of to repair 1/3 of its durability (or 1/4) per piece of material. This should apply to Leather, Hardened Leather, Apiarist's and Cardboard armour.
I have a minetweaker script made for this, but it doesn't seem to work, so I have probably done something wrong: https://gist.github.com/Vexatos/e588fc23cbd5a0e7d30f
Vexatos
commented
9 years ago
You should be able to place a damaged armour piece into a crafting grid with one or more pieces of the material it consists of to repair 1/3 of its durability (or 1/4) per piece of material. This should apply to Leather, Hardened Leather, Apiarist's and Cardboard armour. I have a minetweaker script made for this, but it doesn't seem to work, so I have probably done something wrong: https://gist.github.com/Vexatos/e588fc23cbd5a0e7d30f