I am the guy who did that paint shot you linked to above.
I think you might have mis-understood a part of my post. We don't replace all of the film grain. We go to great lengths to try and maintain as much of the original grain as possible. We certainly add grain to anything we add to a shot but we go to great lengths to match the grain we add to the original grain.
In the example you linked to, I did completely remove the grain from the image before I started to paint out the wire (the painting softens and blurs the grain). But I then took only the areas of the image that I changed with the painting, and but that over the original image, with the original grain on. I then only added 'fake' grain to the areas that I had painted.
The grain that we do add to the visual effects isn't usually 'fake' either. The visual effect supervisor usually takes the cameras that production was using on set and shoots grain samples on the same film stock. We then add these samples back to our VFX work so they match the original footage.
We all so have to do exactly the same with footage that comes from digital cameras but this is more 'noise' than grain. And if the footage came from a compressed source like a GoPro we have to match compression artifacts.
I think you might have mis-understood a part of my post. We don't replace all of the film grain. We go to great lengths to try and maintain as much of the original grain as possible. We certainly add grain to anything we add to a shot but we go to great lengths to match the grain we add to the original grain.
In the example you linked to, I did completely remove the grain from the image before I started to paint out the wire (the painting softens and blurs the grain). But I then took only the areas of the image that I changed with the painting, and but that over the original image, with the original grain on. I then only added 'fake' grain to the areas that I had painted.
The grain that we do add to the visual effects isn't usually 'fake' either. The visual effect supervisor usually takes the cameras that production was using on set and shoots grain samples on the same film stock. We then add these samples back to our VFX work so they match the original footage.
We all so have to do exactly the same with footage that comes from digital cameras but this is more 'noise' than grain. And if the footage came from a compressed source like a GoPro we have to match compression artifacts.