Years ago I tried regalvanizing with a group of friends using a commercial place. I will not do that again! The galvanizing company apparently did not tumble enough after dipping, AND the fellow who picked up didn't check so...
A few years later I was due for more chain. Because the bitter end of about 40' seemed okay, I decided to clean, treat and reuse that portion for my Stop anchor.
I followed the "sure-fire" expertise of an online expert. First I treated/scrubbed with phosphoric acid (ospho, not the green crap that doesn't work) then rinsed with fresh water. Dried thoroughly. Following that I sprayed with galvanizing paint multiple coats.
It rusted anyway, before the chain made it back to Seaweed. Quite frankly, I would replace the chain if at all possible. Because G4 is stronger I would opt for that versus triple-B. Of course these are boats: G4 differs in dimensions from BBB. Good luck.
As long as I'm offering free advice, paint your chain while you've got it out. Above is the chart I used for my 150' of chain. Because I boat/anchor in shallower waters I actually use about 75' of chain maximum. This chart reflects that AND allows me to swap end for end easily.
Yes, I know chosing red-white-blue is not the true nautical old salt version from my youth. It is however for me intuitive. So paint your chain while it is easy.