Helped a friend paint her kitchen last week one night, it is quite a deep red colour. We were all finished, cleaned up and sitting having a beverage and WOW, we notice that we can completely see a darker stripe where the wallpaper border used to be. We had washed the walls before we started and there didn't appear to be any residue leftover. Would this be the glue/paste and how do we get rid of it? The kitchen is completely painted now.