You can take some weapons to the Weapon Refinery, where you can grant them new abilities or increase their power.
Why, that's perfect! I was just thinking that it was time to give my weapon a little boost.
Oh, really? What were you thinking of doing with it?
I was thinking utility—like attaching mirrors to the shaft so I can see behind myself!
That's a...novel idea, Sharena, but I don't think that's an option!
The Weapon Refinery
The Weapon Refinery is accessed from the Ally Growth menu. Here, you can spend materials and SP to strengthen weapons and change their abilities. Note that you can't refine all weapons, but version updates may make additional weapons eligible for refinement.
Prerequisites for Refining Weapons
To use the Weapon Refinery, you'll need an ally who has a weapon that can be refined. Be aware that once refined, weapons can't be inherited, except for when merging allies. Additionally, the Weapon Refinery option will only become available once you've cleared The Rite of Blades, an Intermission chapter in Book I of the main story.