Check out the various Brownell spray on/baking lacquers. They have worked great for me on refinishing aluminum parts. Very easy to use when you follow their instructions.
It will reduce value of a collectible flat top. How much is a good question. But it will more likely make the difference between a sale or no sale at full collector value and reduce to a shooter value. That can be a few hundred bucks.
Another option would be to shop for a replacement grip frame with original anodizing to maintain value. They do show up from time to time. These weren't fitted to the frames and had a generic fit so will likely fit as well as the original.
Assuming you mean an Old Model Flattop, I'd think it would be at least as much as it would cost to find/buy a replacement original anodized grip frame in the desired condition. These XR3 grip frames can easily run $100 or more in "collector" condition.
