The people who lived in my house before me had a dog which has scratched areas of the walls in the kitchen. The walls were painted in matt emulsion and I have since re-painted. The scratch marks only appear to be surface scratches and when I re-painted they disappeared, however they have now reappeared through the new paint and show as a dirty brown mark. On closer inspection you can still see the scratches. What can I use to make them disappear completely?. I have tried rubbing the wall down with sandpaper but it doesn't seem to make any difference. All suggestions greatly appreciated. Thanks in advance