Question About improvement materials

[shi-foo ODIO LA LPF]

Arriving at act 3, I have a lot of very good armor to improve, but I realized that I didn't have many supplies, instead, I have a lot of iron to sell (approximately I would have 3800 supplies in total if I sell everything), in the description when selling it it says that It is used to improve the katana sakai and the tanta sakai, is it true that it only serves for those two things? Is it not necessary for any other improvements in the game?

It seems ironic, but I haven't realized if it was really useful for anything other than improve's the katana hahaha, I've always improving it without looking.

[Drake Ravenwolf]

I keep selling any resouce going over 200 - you stop being able to pick up more at 500, doing this I have all the upgrades I currently have access to and ~12k supplies left over (I'm currently at the start of Chapter 2 but just finished Iki)