I'd strongly advise that you don't use plaster externally. A sand and cement mixture will do the job. To get a smoother finish, use a finer grain of sand and gently smooth over with a float. Paint with masonry paint which you should be able to get from a large DIY store. Usually this is water based.

If painting over bare sand and cement, the first coat should be a primer coat of paint mixed with water. Always read the label – the paint manufacturer's advice should guide you as to the proportion of water to paint to mix up.