Get the wrong one on the wall and the skim either goes off before you can work it, or delaminates a fortnight later. This guide sets out the three jobs, which background needs which, and the two PVA rules that account for a large share of failures in this category.
Priming Is Three Separate Jobs
Job One, Suction Control
High-suction backgrounds pull the water out of plaster before it can be worked. Old lime plaster, aerated block, dry brickwork and grey undercoat all behave this way. The plaster goes off on the wall rather than under the trowel, and no amount of skill recovers it.
The traditional answer is PVA, applied diluted, left to dry, then a second coat plastered onto while it is still tacky. Where the suction is too severe for PVA to cope, the escalation is a dedicated high-suction primer or SBR.
Job Two, Mechanical Key
The opposite problem. Concrete, painted walls, tiles, MDF and dense block are low-suction and smooth. They are not drinking your water, they are simply offering nothing to grip.
Sealing a surface like that with PVA does not help, because sealing is not the issue. What the background needs is texture. Grit primers carry an aggregate that dries to a rough, keyed surface the plaster can bite into.
Sika's guidance on Blue Grit is direct on the comparison: while PVA is sometimes used for sealing, Blue Grit offers superior grip on smooth surfaces thanks to its aggregate content, does not require dilution, and provides a mechanical key that PVA does not. It is ready to use, dries in approximately six hours at 20°C, and the blue colour makes it obvious which areas have been covered.
Job Three, Preparing Finished Board For Decoration
A drywall primer is doing neither of the above. It goes onto jointed and sanded plasterboard, before painting, and its job is to equalise the suction and texture between the board face and the jointing compound. Without it, paint dries to a different colour and sheen over the joints than over the board, which is the patchy flashing effect you see on skipped prep. It also protects the plasterboard surface from UV degradation before decoration. Coverage is typically up to 15m² per litre.
This is a decorating product, not a plastering one, and it is not a substitute for either of the two above.
The Decision Table
| Background | Problem | What it needs |
|---|---|---|
| Old lime plaster, dry brick, aerated block | High suction | PVA diluted, or high-suction primer / SBR if severe |
| Painted wall, concrete, tiles, MDF, dense block | No key | Grit primer |
| Sand and cement render | Moderate suction, some key | PVA, or damping down with water |
| New jointed plasterboard, before paint | Uneven absorbency between board and joints | Drywall primer |
| Insulation board, before render | Dust and low key | System-specific primer from the render manufacturer only |
Render systems are certified as complete kits, and the primer is part of the kit. Substituting a general-purpose primer into a certified render build-up puts you outside the system. Use what the render manufacturer specifies, and nothing else.
The Two PVA Rules
- Plastering onto fully dried PVA fails. The bond relies on tack. Once PVA has gone off hard, it is a glossy film with nothing for the plaster to grip, and the skim will come away as a sheet. If it has dried out before you get to it, apply a fresh coat and work to that.
- PVA is not a primer under cement-based products. It re-emulsifies with moisture, so putting it under a cement render or screed reintroduces the failure you were trying to prevent. Cement-based work needs an SBR or a system primer, not PVA.
Neither of these is a quality issue with PVA. Used for what it is for, on a high-suction background, tacked in rather than dried out, under gypsum rather than cement, it does its job perfectly well and costs very little.
What Actually Goes Wrong
Five failure modes, all of them preventable at the prep stage.
The single most common cause of adhesive and plaster failure. The product keys to the loose surface layer rather than the wall, and takes that layer with it when it goes. Brush down and vacuum before anything else happens.
High suction, not controlled. The wall drank the water.
No mechanical key, or a grit primer used where suction control was needed, or a sealer used where a key was needed. All three produce the same result.
Covered above, and worth repeating because it is so common.
Gypsum products need a dry background and temperatures above roughly 5°C. Priming a cold, damp wall in February does not change what the wall is.
Priming Is Only Half Of It
Most of the walls being primed are about to receive plasterboard, insulated board or a skim, so the adhesive choice sits alongside the primer choice.
A few points that come up repeatedly:
- Bagged gypsum adhesive is non-combustible, Euroclass A1. Foam adhesives, including those sold as "FR" or fire retardant grade, are Class E to BS EN 13501-1. Both have their place. They are not equivalent on fire.
- Foam is fast, gypsum corrects. Foam adhesives cure using moisture from the air and substrate and get boards up quickly, but the bed is thin, so an out-of-true wall stays out of true. Bagged gypsum adhesive gives you a thick enough bed to level the background.
- Never adhesive alone on ceilings. Mechanical fixings are required.
- Watch solvent grabs near insulation. Solvent-based grab adhesives attack EPS and XPS. Use a hybrid MS polymer or a foam adhesive rated for the insulant.
- Dot and dab needs a perimeter ribbon. A continuous fillet around the perimeter of the wall, services and openings does three jobs at once: airtightness, cavity barrier at the board edges, and closing the flanking path behind the board. Dabs alone leave an open cavity.
A Note On Dust And Skin
Gypsum and cement powders are alkaline and irritant, and mixing dry product from the bag is a respirable dust exposure. Cement-based products additionally carry a chromium VI dermatitis risk and can cause cement burns on prolonged skin contact. Gloves, eye protection and a mask when mixing, every time.
If you are using a PU foam adhesive professionally, note that products containing more than 0.1% monomeric diisocyanate require the user to have completed diisocyanate safety training before use. That has been a legal requirement since 24 August 2023 and it applies to employers and the self-employed, not to DIY users. Training is available through the industry programme at safeusediisocyanates.eu. Once the foam has fully cured the diisocyanates have reacted, so the hazard is at application rather than in the finished wall.
Not Sure Which One You Need?
Work out which of the three jobs your background presents, and the product choice follows. High suction, seal it. Smooth and low suction, key it. Finished board before paint, even out the absorbency.
If you want to talk a specific background through before ordering, call the team on 03003 034 578, or browse the full adhesives and primers range online.
