Board Thread:Questions and Answers/@comment-44835548-20200110171953/@comment-36170150-20200110192819

Yes, you can do the Premium Steel custom equipment quests at any point after you invest in it.

With Robin's crystal you will need to purchase a crystal from the Yhilini Order of Thaumaturgy (it will be more expensive during chapter 4, 10000 Sx).

With Varia's blades you will need to purchase steel from Eustrin, Eustrin does get locked away near the end of chapter 4 so there is a deadline to do it if you want it before Chapter 5 (it will probably be possible to get it during chapter 5 anyway)

With Altina's wand you don't need Premium Steel at all because all her requirements are met by chance while the story progresses.

A few new custom equipment options become available near the end of Chapter 4, and those DO have a non-neglegible effect on the war at the end of chapter 4, so the latest you can invest in Premium Steel without missing anything critical is before the war in Chapter 4.