Plaster is best primed with an alkyd primer. Since you have an adhesion problem I'd suggset that you thin the primer about 25% with mineral spirits. I'd also suggest that you use XIM as a primer. Expencive stuff, but it sticks to anything. XIM is thing as it is, so you'll need to protect the surroundings well. Next you need to apply an HIGH QUALITY Latex paint that has the ability to breathe. Usually, breathability is a property of exterior grade paints. Consult a local retailer about applying exterior paint in your porch.
As Gordon elluded to, it may be a good idea to find a way to insulate your porch.