So I live in an older condo with Concrete walls. I was sanding some rough spots and previous owner poor patch jobs in preparation for a new paint job. However, a ton of paint simply started peeling off the walls and I am now left with something that looks like: http://i.imgur.com/qhSNH.jpg. Here is a close up:http://i.imgur.com/egt9y.jpg . Now the paint left on the wall will not lift or scrape off easily, leaving a significant edge between the two areas. What is my best course of action to smooth this out as I am sure painting over anything like this will leave visible lines.
You are right- you can't just paint over it.
There's a reason that the other paint didn't adhere- is the mud ( i assume) that's left real powdery?
I would stabilize the wall with a thin penetrating primer called Gardz- do a search here on it for some application tips-
It will glue everything down-
Then the wall needs to be skimmed with joint compound- might take 2 to fill enough- then sand smooth- then reprime, then paint 2 coats.
All these steps are searchable- but it might not be a bad idea to get a pro to do it- it isn't easy if you haven't done it before.
"It's better to come here with questions before you screw up than to come here after and ask how to fix them."- JS