what I would now do is to clean the surface real good, do any sanding necessary, then put on a good quality paint.
I normally spray surfaces like this and for one reason. You know when you apply paint with a brush you get what we call gutters? Gutters that run parallel to the ground will catch dirt in the trough and look bad, but if you brush downward you have the gutters going down and the debris just washes off.
Spraying will eliminate all gutters. With a brush, flowtrol mixed with paint acts like a self levelor and will also heal the gutters.