I would always prime the surface of new or unpainted plaster.
This is best done using a plaster primer or a dilution of emulsion paint, diluting it with 10% water, so a ratio of 9 parts paint to 1 part water.
Thinning just 10% for mist coat priming bae plaster is not enough IMO & risks poor adhesion & problems with cracking/peeling in the future. I thin matt white emulsion (own brand) 25-30% & apply 2/3 coats in quick succession, dry overnight & then your chosen paint finish.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local