I recently bought a new flat, approximately 50 years old construction. One of the rooms had wallpaper which I took off with wallpaper stripper. But when I was taking off the wallpaper, some of the old paint (green) came off along with the paper. After I tried to scrape the paint and sand papered it to make the wall smoother. Then I applied the first coat of brilliant white silk emulsion, when it dried, I can still see the patches of old green paint through the new white paint. I applied second coat but still it didn’t solve the problem. Any suggestion please??