Surface preparation is a must before painting because the total life of a paint coat depends to a great extent on how carefully the surface preparation is done.
   

1 Surface should be thoroughly cleaned by wire brushing or coir brushing to remove all loose particles.
2
Treat areas of oil, grease, moss or mould infestation with cleaning Solutions such as soap, hypochlorate solutions etc. Wash thoroughly with clean water to remove residues.
3 Brushed surface should be thoroughly washed with running water to remove all dirt, dust, loosely adhering particles.
4 Allow to dry out thoroughly before application of the paint.

5 Repair any cracks, holes or other imperfections with suitable fillers.
6 After the filler dries, sand to smooth finish, with a damp clean rag, wipe away all the loose dirt and dust..
 
Note: The surface should be perfectly dry prior to painting. Otherwise this paints may tends to blister or peel.
 

Read more - Painting Tips