Yes I agree you just need to buy some of the cheapest white emulsion you can, then water it down a little. This will seal the plaster really well. If you don't do this and you just put the paint straight on to new plaster you will find that when you come to put the second coat on it will pull the first coat off, this creates massive problems. If you use white watered down emulsion then you will see shadows where there are small marks, you can then sand these before applying the final coat of paint. This will result in a perfect finish.