Just in process of trying to tidy up a spare bedroom. It was painted with a hideous purple silk emulsion, and was a good smooth finish. We have painted over it with good old matt magnolia, and in some places there are little bubbles starting to appear. All walls have now had 2 coats of matt, and whilst one wall bubbled after 1st coat, another has only done it after the 2nd coat! It seems to be the purple paint that's bubbling rather than the magnolia, but no idea why this is happening and why it's only in certain places in the room. Any suggestions would be welcome, both on why it's happened and how to get rid of them (I assume just sanding them off and repainting again?). Thanks!