Why is my mist coat not sticking to plaster? This might be down to a couple of things. There might be dust on the walls, creating a barrier. Your plaster might be too polished, in which case you might need to abrade it, or go with a different primer.
If the plaster still has moisture in it prior to painting, the paint may attempt to bond but the surface of the plaster is weakened by the already existing moisture and the paint will flake off. Hence, the problem that you are having with seeing plaster on the back of the paint flakes.
The majority of mist coats dry to the touch in two hours or less. But you should give yourself at least 24 hours for the cure. As a result, the topcoat would attach better to the substrate because the paint would have had enough time to cure completely and bond with the plaster.
You need to rough up the surface with a rasp, or deliberately create a rough surface as you create the first layer. Then, when you're ready to add more plaster, you need to wet the surface you're going to work on. Otherwise, it pulls moisture out of the new plaster too quickly, and it doesn't set properly.
If you have some PVA glue put some in a bucket and dilute with water at a ratio of 50 parts water:1 of PVA. Brush onto the fresh plaster and allow to dry. Then put a coat of paint diluted 1:1 with water. That hopefully should cure your problem.
Preparing new plaster for painting
Applying a mist coat can be messy as the paint is thinner than usual. Lightly sand the plaster with 120 grit sandpaper to remove any small imperfections and to key the surface. If the plaster is too shiny the paint won't adhere.
What Causes It? Peeling from plaster could be a result of insufficient wet troweling of the white coat when the plaster was originally applied, causing chalking of the surface. Very hard plaster may be slick, reducing the adhesion of any coating.
Apply High Quality Primer
Old plaster walls are better off with an oil-based primer. This is because oil primers are far more superior at preventing any stains from water and other materials from showing through, which is a more commonly found problem with plaster walls.
Slightly wetting the wall can help the plaster adhere better, but avoid over-saturating it, as this can weaken the plaster's bond. Damp proofing walls before plastering is a vital step in ensuring a durable and moisture-free finish.
Wash the wall thoroughly with soapy water or wall cleaner (grease and dirt can result in paint not sticking to wall) Add filler to any cracks or holes, and sand the area until smooth. Prime the wall using a water-based wall primer.
Leave the mist coat for 24 hours to dry. A simple test to see that the mist coat has worked is to use masking tape. If it comes off the wall clean, you're ready to go.
All you need to do is dilute some emulsion paint with water and apply it to the plaster using a roller, paintbrush or spraying. One mist coat should suffice, but you can apply a second (or even third) coat just for extra peace of mind.
Most probably, the pva adhesive has been mixed with a lot of water before it has been given to the wall surface, another factor that produces the splitting of the portion is the thickness. Normal plaster must have a thickness of at least 4 mm, when the thickness is thinner it can easily detach.
Due to the fact that newly plastered walls cannot be painted using certain paints until they are completely dry, contract matt is the best paint to use. This is for a number of reasons, which include cost and moisture.
New plaster is porous, so if you paint it with standard paint it will simply be absorbed by the wall. This means you'll need to apply more coats of paint, and the paint could start to peel later as it dries. To prevent this happening, you need to apply a mist coat first.
This is because the moisture interferes with the adhesion process and prevents the plaster from bonding to the surface. For this reason, it is important to make sure that you apply the plaster to a clean, dry surface. Remember, simply replastering or painting over damp walls only treats the symptoms and not the cause.
A ratio of 1 part glue to 4 parts water is usual. Roll or brush the glue onto the wall. Don't wait for the glue to dry completely. Once it is a little sticky (or “tacky”) to the touch then you can start applying plaster.
Painting on wet plaster is not recommended. If the plaster is not fully dry, painting over it will trap moisture between the plaster and the paint, eventually causing the paint to flake and peel off.
Let the plaster dry
Your plaster will dry quicker with a heater or if you open a window. If you try to apply paint to wet plaster this can cause adhesion problems. Applying emulsion to wet plaster also means that it may not bond properly and you could find that it peels off your wall.
Once the plaster has been repaired, you will then want to paint. You should still apply a mist coat to the old plaster.
Light sand and brush
Freshly-plastered walls should be smooth on the whole but you may find the odd plaster splash or pieces of dust that have been left. Use a fine sandpaper and a brush to remove these blemishes but don't be too vigorous with your sanding.
You'll find a few different ratios of water-to-paint floating around online when it comes to your mist coat. However, a common ratio is 3:1 – 3 parts paint to 1 part water. Having said that, before you mix up your mist coat, make sure to read the information on the back of your paint tin.
Leave the plaster to dry
There are different factors that influence how soon can you paint on new plaster, such as the type of material and weather. As a rule of thumb, plaster takes up to 4 weeks to dry completely. You can speed things up with a heater or by opening a window.
USG Plaster Bonder: Pink is a vinyl acetate homopolymer emulsion used to bond new plaster to any structurally sound interior surface. It provides enhanced plaster adhesion to surfaces like gypsum plaster, cinder block, stone, drywall panels, and other similar materials.