It looks like you are sanding off the coating from the coil brackets? I'm not sure exactly what the factory brackets are plated with, but it looks like a zinc coating of some kind obviously meant for corrosion protection. Once you sand the plating off those brackets will probably start to slowly rust from just humidity. Are you spraying them with a clear of some kind?
Also, I can see a lot of sanding marks on your brackets and they look pretty rough. You may get a better looking result cleaning and then buffing on a wheel for the same amount of effort.