Currently painting a new build with a couple of coats of Dulux supermatt to kick off the decorating process. Have noticed that some walls have shiny patches in the plaster that is proving very hard to cover with paint even after 2-3 coats. The problem seems to be worse where block walls were scratched and skimmed as opposed to walls that were drylined and just skimmed afterwards. The plastering was finished months ago and the house is dry. Does anybody know what causes this or have any suggestions as to how to cover it?