What to do with old gear?

I've just reached level 30 and I'm starting paladin quest.
But I was wondering what I should do with old gear I've a 100% spiritbond with?
Convert it all to materia or is that useless on low lvl gear?

If it's convertible convert it. Either sell the materia on the marketboard or keep it for later use.
The rest you can eiter trade in at your grand company, sell it to a vendor or if you like the design keep it for glamor.

Forgot one: You might also use it to practise desynthesis

Also if you consider leveling a Warrior sometime in the future, keep some of it (especially the more rare ones - green and pink), it will help.