Renew GizFab 4-2-1?

Bit of a weird question but does anyone know how I can clean up my 4-2-1?

It used to look really nice when I first got it, but now it doesn't have the same look or shine at all.
It looked nice until my engine swap was carried out and then after, it just never looked the same again.
I have tried cleaning it rigorously but no luck, almost as if it is stained!

Any ideas?

I know you can buy exhaust manifold cleaner which is supposed to remove any blueing tint etc but unsure if its just snake oil or whether it will actually remove whatever is on the manifold?

Before: (ignore the bluing on cyl4, this was caused by cyl4 running hotter due to a faulty injector)





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