first power wash the entire area with a good psi washer and with a nice fan tip.
Let it dry thoroughly.
Go to a reputable paint dealer and describe what your doing and the surface. You might want to get an additive called flotrol that assists in adhering to chalky surfaces and is a self levelor too and has a primer with it. you will have problems applying this if by brush and you will see brush hair lines. A roller will do the same also. I would higly recommend in spraying this on with a good sprayer.