You only have the genuine patina once, the result of years of weathering - I guess what you are trying to do is stop the hands of time and preserve it in its current state....
By sanding it, I expect you are wanting to smooth the edges to keep it from flaking ?? What is likely happening is the paint is gradually shrinking and drying out at the cracks, moisture, etc., gets under the edges so it curls up and breaks off. What you need to do is seal the surface somehow, like with a flat clear coat. Problem is that when spraying it on, it's very tough to get enough in the cracks to seal it well unless it is on a flat surface so you can lay it on heavy enough without running. Or is it just me that when I spray clear, theres gonna be runs...[S