If your walls are newly plastered, here’s what you need to know before painting over them.
1. Seal it
You first need to seal new plaster to make it less absorbent and to help the topcoat adhere better.
A common way to seal it is with watered-down emulsion (known as a mist coat), as the plaster sucks up the water and becomes less absorbent. After you've applied the mist coat, you'll be able to see where you need to fill more easily than you would with bare plaster.
2. Use watered-down emulsion
Watered-down emulsion is messy to work with because it drips much more than standard emulsion. Be careful to wipe, roller or brush-out drips immediately to avoid a bad finish - the paint dries quickly because the plaster's absorbent. If your topcoat's white, it's best to use watered-down white emulsion for the mist coat, or you may end up doing more coats of topcoat than you need to.
3. The trouble with white topcoat
Another problem with using a white topcoat on new plaster is that you can get patches of plaster the topcoat takes several coats to cover. To save time and paint, use a stain block or a basecoat emulsion on these patches. Ronseal One Coat Triple Action Basecoat is specifically designed for new plaster (dilute with 20% water for the first coat) and problem walls. It seals the plaster and also fills hairline cracks, which sometimes appear in newly plastered walls and ceilings, especially if they're lath and plaster.
4. Specialist paints
Paints designed to be applied directly to bare plaster are available in DIY stores. Although they're more expensive than watering down cheap emulsion, they're much nicer to use because they don't drip everywhere, but it can be harder to get a good finish with them.
Watered-down emulsion produces a 'soft' edge on new plaster, whereas bare plaster paints often produce a 'harder' edge that can adversely affect the finish, so it is advisable to water down the first coat, if you can cope with the mess. Painting plaster before it's fully dry can cause the paint to peel, giving you endless problems, but some bare plaster paints allow the plaster to continue breathing and drying after the paint's applied.
5. Dealing with damp
Sometimes patches of new plaster don't dry out because of damp. Often the best solution is to remove the plaster back to the brickwork and get a plasterer to do a waterproof render before replastering. This should stop any moisture in the brickwork coming back through the plaster. The cause of the damp should also be addressed.
Alternatively, there are quick fixes, such as applying damp paint/seal to the damp patches and then painting, or tiling or cladding the wall (with tongue-and-groove panelling, for example). However, you're covering the damp rather than dealing with it and it may come through again once the damp paint/seal starts to fail.