Occasionally, I work on these old houses from the 40's and they have plaster walls. There will be a few cracks in the paint. I go to scrape off the loose stuff and I find myself scraping off the whole wall! Under all the layers of paint, there is a really smooth, shiny surface. It's not surprising that paint has trouble sticking to it. I don't think that surface is paint. I think that is the actual plaster. But why is it so smooth and shiny?