We removed a large bathroom mirror, and had the wall professionally painted (well, if you call what they did painting). The wall looks awful because you can still see the mirror outline. I am guessing due to several coats of paints over the years (about 30 years old). What is the best (and hopefully easiest) way to smooth this out?
The easiest way to deal with the lines is to apply drywall mud to cover or hide the lines. At least this is how I would deal with the problem.
Doing this has its own set of problems; prime before applying the drywall mud or risk adhesion problems, will require at least 2 coats of mud, carefully sand paying special attention to the edges of the "patch", and the area might need some texture or imagination to get it to match the surrounding wall surface.
