Do I need to PVA a wall before plastering?

PVA is the most over-used product in DIY plastering and also a genuinely useful one, which is why the advice is so contradictory. The confusion goes away once you separate the two different problems a background can have: too much suction, and not enough key. PVA addresses the first and does very little about the second.

What you'll need

• PVA or a proprietary plasterers' bonding agent
• a bucket for dilution
• a brush or roller
• a bonding agent with aggregate, for smooth surfaces (optional)

Step by step

1. Work out whether the background has a suction problem.Splash water on it. If it vanishes instantly, suction is high and plaster applied to it will go off far too fast to work and may craze or fail to bond.
2. Work out whether it has a key problem.A smooth, dense, non-absorbent surface — painted plaster, dense concrete, old gloss — has nothing for plaster to grip. That is a different problem and a coat of PVA does not solve it.
3. For suction, dilute PVA and apply the coats the way plasterers actually use it.Typically a well-diluted first coat allowed to dry, then a less diluted second coat that is still tacky when the plaster goes on. The plaster is applied to a tacky coat, not a dry one — that is the part most often got wrong.
4. Consider a proprietary bonding agent instead.Plasterers' bonding agents and SBR-based products are designed for the job, do not soften when damp, and are what most professionals use in preference to general building PVA.
5. For a key problem, use a bonding agent with aggregate in it.Grit-filled bonding primers give a mechanical key on smooth or non-absorbent surfaces. That is what makes plaster hold on a painted or dense wall.
6. Do not use PVA under tile adhesive.Different job, different rules. PVA can re-emulsify when it gets damp, which under tiling in a bathroom is a failure plane exactly where water arrives. Use the adhesive manufacturer's primer.
7. Do not use PVA anywhere that will be permanently damp.Below a damp course, in an unheated outbuilding, on a wall with a moisture problem — the film is water sensitive and the wall's problem needs fixing rather than sealing.
8. Remove dust and loose material first, whatever you use.Any bonding agent applied to dust sticks the dust to the wall and nothing else. Brush and vacuum first.

Tips

• Time your PVA to the plastering, not to your convenience. A coat that has fully dried has lost much of its point; you are aiming to plaster onto a tacky coat.
• New plasterboard usually needs nothing at all before skimming — it has a paper face designed to take plaster.
• If the wall is so friable that a brush takes material off, no bonding agent will fix it. That surface has to come off.
• Do not use PVA before painting either. That is a separate myth, and a mist coat of thinned emulsion is the right answer on new plaster.

Common mistakes

• Applying one thick neat coat of PVA and letting it dry hard — It forms a plastic film on the wall. Plaster applied to a hard dry film has no suction to help it grip and no key to bite into, so it can slide, and in the worst cases the whole coat debonds later.
• Using PVA on plasterboard before skimming — The board's paper face already gives correct suction and key. PVA reduces suction to almost nothing, which makes the plaster harder to work and can leave it drying unevenly.
• Using PVA as a bonding agent on a painted or dense wall — There is no key. What holds the plaster is a film of PVA sitting on a film of paint, which is two weak layers stacked. A grit-filled bonding primer gives the mechanical key that is actually needed.

If it doesn't work

Questions people ask