I replaced the original calipers on my AT with a newer set I bought off eBay, but the originals are still in quite good condition. However, it's clear that the unpainted bits on the inside are corroding and I'm wondering if I can protect them by painting these bits. The only worry is that this will require painting right up to the edge of the pistons, and ideally include around the lip of the cylinder, which is where I understand the calipers usually start to fall apart.

Apart from the obvious practical difficulties of not getting paint (or dirt) into the cylinders, is this likely to be useful or just a creative waste of time?