Deep color overlaps worse. It also depends on what color it is. If it is a common painting, there is no need to scratch. If there are already more layers, I recommend scraping. If you are going to cover the rich paint, I recommend the first layer much thinner / expect to apply the paint 2x-3x / This applies to basically every paint. It's best to scrape it when you soak the wall with water. He wants to brush the painting several times in a row and then scrape it off with a painterly, flexible spatula. If the walls need to be repaired, cover larger holes with plaster, for smaller unevenness, putty is sold, or the final plasterboard paste can be used. After drying, regrind and you can paint. Wash the greasy stain with water and detergent, then several times with clean water. If it is deeply soaked, it would be advisable to scrape. I always recommend a paint brush, not a roller, for applying the paint as well as soaking and washing. ****** better incorporates the color into the wall, they peel off and the painting lasts longer.