I've heard of 'saponification', but what exactly is it?

Saponification occurs when oil-based paints are softened and liquefied by the alkali in plaster when moisture is present. Materials containing Portland cement or lime are strongly alkaline; gypsum plasters are usually not, but can become alkaline if gauged with lime or if it is brought forward from the backing during the drying phase. For these reasons, never use oil (solvent) based paints on plaster or masonry surfaces until they are completely dry in depth, then prime the surfaces with Dulux Trade Alkali Resisting Primer. Where saponification has occurred, completely remove the defective coating, wash down the surface and rinse with clean water. Allow to dry then prime with Dulux Trade Alkali Resisting Primer.