Painting preparation is the most important part of any painting job. If you get it right it can make life very easy, but if you get it wrong it can be an absolute disaster.
The big question is where to start?
It depends on what substrate you are painting.
Here is a list of the main surfaces you will be painting on, however there are many others.
I will say it again, preparation is the most important part of any painting project whether it is a new or repaint job. You need to make sure that the surface you are painting is sound and has no contaminants or dust.
When repainting, it can even be worth doing an
Adhesion Test to make sure the under layers of paint are sound.
Be aware of Lead Based Paint and Asbestos in older buildings.