So, the Eastwood stuff is an acrylic paint which looks like zinc chromate. I wonder how closely it will match?
If you are going to remove the existing zinc with carbide paper or whatever, I guess that will get you down to to plain steel. As you don't have endure the under-bonnet conditions of temperature and petrol, I wonder if you could find something cheaper than £16, perhaps something from the model paint people like Humbrol (who have a metallic bronze).
I'm not too sure that any paint will give a really close match to real zinc plate, but guess it depends on how demanding you are
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B