Hi, I am about to paint the exterior walls of my house, the front and side are in good condition just need a clean up before painting, but the back of the house which is just the upstairs as I have a flat roof extension is in quite a bad state ( see attached photos ) although it looks as if the paint would scrape off easily it doesn't, when scrapped very little comes off and seems to get flattened to the wall. What would be the best way to tackle this, was thinking some sort of sealer. Thanks for any advice