When you finish one part of a job, it can be tempting to rush into the next part while you are still going strong. Unfortunately, when it comes to decorating this tends to cause more problems than it solves, resulting in longer and more drawn out work rather than an early finish.
This is definitely the case when it comes to painting plaster. You cannot just paint over a newly plastered wall, as the plaster will absorb the moisture of the paint, drying it out and leaving it more likely to flake and peel. This in turn will mean having to repaint far sooner than you would otherwise.
Instead, you will first need to seal the plaster with either a ‘mist coat’ or PVA. You can prepare and apply this yourself, but as with most jobs which can be done with DIY, your best guarantee of a fantastic finished product will be to call a professional. This work will best be left to a local painter decorator, especially if you are working on several freshly-plastered rooms at once.
The reasons for sealing plaster are twofold: first, fresh plaster is porous and absorbs moisture, so if you apply paint directly it will become dried out and ugly. Secondly, sealing the plaster will help the next coat of paint to adhere properly to the wall.
How to do this will depend on the paint you are using. For any non oil-based paint, you would use a ‘mist coat’ made by mixing water and emulsion, while for types like gloss or eggshell you would use a mixture of water and PVA. There are brands of paint which are designed specifically for fresh plaster, but they can be quite expensive.
When creating your mixture, you will want to use lightly coloured emulsion. If not, the mixture will be more likely to show up through the next layer of paint. You should also ensure that the emulsion you use is non-vinyl, as otherwise a skin will form on the surface of the plaster.
The ratio for your mixture will usually be four parts water to six parts emulsion/ PVA. You can add more water if necessary; remember, you want it to be diluted, but not so much that it will end up dripping off the wall! Once the components have been thoroughly mixed together, you can start applying the coat to the plaster.
You will not typically need to apply more than one coat, though this certainly will not hurt. When the surface has completely dried, you can finally start painting.