I have a tall wall (16') in my entrance way which was dry walled with 8' sheets and mudded in the middle about halfway up the wall. Over the years this area has jutted out such that you could see a ridge about a 1/2" wide going all the way across the wall.
I repaired this by grinding the ridge down and then mudding the area using a 16" "banana" trowel. I applied about 3 coats and sanded after each until everything was smooth and feathered at the edges - I used my hand to check the feathering. To see what the repair would look like I painted the area with a couple of coats of latex paint. The problem is that I can still see the outline of the edges of the repair under the paint. Although I can see the outline, running my hand over them feels smooth.
Could this have happened because I did not use a primer coat first?
Is there something that I can do to correct this problem?