The damage of a Legendary quality weapon continues to increase past level 100, hence it is worth to use Fortify Smithing enchantments even if the base skill is at the maximum. With the relevant perk and a Smithing level of 100 or above, it is possible to improve a weapon to Legendary quality.
Each upgrade level has a required skill level, and this level can be reduced with the perk associated with the type of weapon that the player wishes to upgrade. There are six levels of upgrade available (from least to greatest): fine, superior, exquisite, flawless, epic, and legendary. However, certain weapons require more exotic materials in order to upgrade them, such as the chitin of a Chaurus for Falmer weapons, or even ingredients normally used in alchemy. Appropriate metal ingots are required to upgrade each item (i.e., Ebony Ingot for an Ebony Sword). Using a grindstone increases your Smithing level, although to a lesser degree than actually forging a weapon.
Grindstones may be found in almost every corner of Skyrim's cities, towns, and even in mines, caves and forts. Unlike in previous games, weapons in Skyrim no longer have durability, making the increase in damage and enchantments the only way to upgrade a weapon.