11. Can I coat a metal surface that has been already coated with another paint?

You can coat any surface that had previously had different product applied to obtain different colour or a different decorative effect. But you should check the following: whether the existing coat flake off, is the contact of coat with surface good, are there any paint "bubbles" which might contain undercoat rust. If everything is alright with the old coat, make surface slightly mat with a fine-grained abrasive paper. Next, follow the basic painter's rule, i.e. make a test (preliminary) painting on a small surface to check if the product is good for use. If an undesirable reaction occurs during the first couple of hours of drying, you should remove the old paint coat and then follow the instructions given on the label of the product.