A quick and easy way to do this inside the blender compositor is to bring in an image texture, and then use a mix node set to divide (for the specific texture I was using) to add it to the rest of the scene. While making my scene I noticed that by doing it this way, the scratches only showed up where there was glare or brighter light, which I found to be close to real life.
You can see the node layout in the attachments below.
You may notice that in this screen shot that around the lens flare that there are scratches.
I have provided two images for a scratched lens effect. Feel free to use them at you own will!
One thing that I’m noticing here is that you are not using lens flares. I am not 100% on this, but I think the reason why this is not working is because you don’t have anything to divide by. Meaning that the image is going to stay as an overlay in a way. So you need to have a divider and a dividee. But I am not totally sure on this. I did try to replicate your node layout and got pretty much the same result. I’m sorry that I don’t have a definite answer for you.
