This might sound very obvious to many people but believe me, hardly anybody does this simple thing anymore – thoroughly wash over all previously painted surfaces which are to be decorated, then rinse off with clean water.
This will add years to the longevity of the finished work and stop many future problems arising. Paint will just not stick to dirty or greasy surfaces, well, not for long anyway.
Use Sugar Soap for this job. It is the decorator’s detergent specially designed for this job and does not froth up too much and so makes rinsing off easier.
Do not use household detergents like “Fairy” or “Flash” which are very frothy and hard to rinse off and also will leave silicates on the surface which will impair paint adhesion.
Sugar Soap can be bought at most D.I.Y. stores but mostly in a concentrated liquid form, which I find too frothy, or from a decorator’s merchants in a granular form (hence the name “sugar soap”) and this is the type I always use.
If you come across any mould or fungi while washing down don’t just wash over it, as this will spread the mould and spores around even more, infecting more areas in time. These areas will need to be treated with a fungicidal wash.