House exterior wall painting-advice needed

Floozie

Can anyone help with painting advice for a house exterior wall.  It was done last year but is already looking very tatty. It is a south facing wall in an exposed area and it was done with Sandtex masonry paint directly painted onto the wall which I think is a rendered wall. When I brush my hand against the bare areas where the paint has come off it feels a bit damp and as if the chalky render residue is coming onto my hand.
Do I need to use a primer before the Sandtex and also with the flaky bits of paint on the wall do I sand them to make the surface more level for when the paint goes on. I have taken the photos so you can see how bad the wall has become. Any ideas/advice most welcome.