The short answer
On highly absorbent, matt and porous plaster, you use a penetrating primer (a deep primer or dispersion primer) that equalises the absorbency and binds the dust. Dilute it depending on how much the plaster absorbs, so that you saturate the substrate evenly without forming a gloss film. If there is a dense, smooth sinter layer, then the logic reverses and you need a bonding primer, as a penetrating primer cannot grip there. And of course: the plaster must be completely dry first.
Why absorbent plaster makes your finish cloudy
Finish absorbs unevenly because the plaster absorbs unevenly. Where the substrate draws a lot of binder away, your paint disappears into the wall faster and dries locally differently than next to it. The result is visible as dull and shiny areas, clouds, streaks and colour differences, no matter how neatly you roll. The paint simply does not have the same amount of time to flow evenly everywhere.
A primer solves this by regulating the absorbency. It saturates the porous areas so that the whole wall absorbs equally strongly or weakly. This keeps your finish open for the same amount of time everywhere and allows it to flow evenly. With fresh plaster, there is an additional function: the surface often dusts and chalks a bit afterwards, and a good primer binds that dust so that your finish adheres properly.
It is important to read the substrate correctly, as not all plaster is the same. Fresh, machine-applied plaster that is matte and porous absorbs strongly. But a smooth polished or machine-smeared plaster layer gets a sinter layer, a thin, dense and sometimes shiny film that absorbs very little. On those two substrates, you work with a different product.
Step by step
- First, check if the plaster is dry. Priming on still damp plaster traps construction moisture, and then you have to start over later. How to measure that reliably is explained in the separate question about measuring a new build wall.
- Then assess the suction power with a simple water test. Apply a few drops of water to the wall. If the water is absorbed immediately, you are dealing with strongly absorbing plaster. If it beads or stays on the surface for a long time, there is a dense or sintered layer, and you need to approach it differently.
- Next, dust off the wall and remove loose or chalking parts. If there is a shiny sinter layer, lightly roughen it first by sanding, so that your primer gets a grip.
- Then choose your primer based on what you have established. On strongly absorbing, porous plaster, you go for a penetrating primer or deep primer based on dispersion. Dilute it according to the manufacturer's instructions and according to the suction power, as an undiluted deep primer on strongly absorbing work forms a glossy film on the surface, and then you have a dense, uneven layer again. Apply it evenly and do not leave any puddles. On a dense, smooth or weakly absorbing plaster layer, you turn it around and use a bonding primer, possibly a quartz-filled one, as it provides grip where a penetrating primer would slide off.
- Allow the primer to dry completely according to the technical data sheet before you start painting.
- Finally finish with a system that fits. Align your primer and topcoat with each other and follow the manufacturer's recommendations, as those two should work together as one system.
The mistake you should avoid
The classic blunder is to apply the same product everywhere blindly without reading the substrate. A penetrating primer on a dense skim coat does not adhere, and a dense bonding primer on highly absorbent porous plaster does not regulate the suction. In both cases, your topcoat will still apply unevenly. So first test the suction and only then choose.
A second pitfall is applying an undiluted primer on highly absorbent work. Instead of equalising the suction, you create a glossy film that the topcoat reacts unevenly to. Diluting to the suction is not a detail but the essence of the matter when it comes to fresh plaster.
What you need for this at PrepKing
Preparation is what PrepKing is all about, and the right primer is an essential part of that. With us, you will find a penetrating primer or deep primer for highly absorbent, fresh plaster, a bonding primer for dense or smooth plaster layers, sandpaper to roughen a skim coat, and foil and tape to cover everything neatly.
No sales talk, just the right product for the substrate in front of you. Every surface, every time.
Related: how to measure if the plaster is dry enough, and for the DIYer how long new plasterwork should dry. Later this year, we will also outline the differences between a bonding primer, a primer, and an insulating primer.
