Definition from Wiktionary, the free dictionary
- An extremely powerful detergent used for washing surfaces prior to painting.
- Sugar Soap Cleaning Crystals are a powerful and extremely effective cleaner used by professional decorators for many years in preparation for painting. Sugar Soap is an easily desolvable, non-caustic, free flowing white powder. Sugar Soap will easily cut through really bad grease and grime, even removing nicotine stains, and makes repainting much easier. Sugar Soap can also be used as a general purpose household cleaner and has many other uses around the home… 
- For previously painted surfaces there is no need to remove all the paint if its is not flaking and is in a stable condition. Clean thoroughly with sugar soap, sand down with a fine paper to give a key for the new paint, and paint. 
- Although modern paints cope surprisingly well over a small amount of dirt, it is far better to wash down with sugar soap first.