If one buys a house (that is already painted) from a non-Jew, he doesn’t have to scrape off a square amah opposite the entrance (Shulchan Aruch, Siman 560). If he wants to paint over the wall (assuming the paint already on the wall is still in good condition), must he leave a blank space (or even scrape off an amah)? If he already painted a new layer without leaving a space, what is the halacha? Thank you for your time.

He must leave a blank space only if he scraped off the old paint before putting on the new coat.