Should I seal a dusty or chalky surface before gluing or sealing it?

Adhesion is a chain, and the weakest link is usually not the adhesive. A grab adhesive bonded to a chalky wall holds perfectly and the chalk lets go; a sealant bead on old render pulls the render off. In both cases the product performed exactly as advertised and the surface did not, and the surface is the part you can do something about.

What you'll need

• a stiff brush and a vacuum
• a stabilising primer or an acrylic primer suited to the surface
• PVA, for the narrow cases where it is appropriate
• a scraper, for anything loose
• a cloth and a bucket of clean water

Step by step

1. Test the surface first.Wipe a hand across it. Scratch it with a fingernail. Press a strip of tape on and pull it off. Powder on the hand, a groove from the nail, or dust on the tape all mean the surface is friable and nothing should be stuck to it as it is.
2. Remove everything loose before considering a primer.Scrape and brush off flaking paint, blown plaster, loose render, wallpaper paste residue and surface dust. A primer stabilises a dusty surface; it does not glue a loose one back on.
3. Understand what a stabilising primer does.It soaks into the friable layer and binds the loose particles together into a coherent surface with the sound material behind. It is a different product from a bonding primer or a sealer, and on a genuinely chalky exterior surface it is the right one.
4. Use an acrylic primer for ordinary porosity control.Where the surface is sound but very absorbent — new plaster, plasterboard, aerated block, bare timber — a thinned acrylic primer or the manufacturer's recommended primer evens out the suction so the adhesive or sealant does not lose its water or solvent instantly.
5. Know when PVA is appropriate, which is narrower than tradition suggests.Diluted PVA is a reasonable dust-binder and suction control under some decorating work and under some solvent-free adhesives. It is not a bonding agent under tile adhesive, it is the wrong product under plaster in most cases, and it re-emulsifies when wet, so it should never be used anywhere that will get damp.
6. Never seal a surface that needs to dry.A wall that is damp, or new plaster that has not dried, must not be sealed. Trapping moisture behind a primer or an adhesive causes blowing, mould and a failure that takes the surface with it. Find and fix the moisture first.
7. Let the primer dry fully before bonding.Tacky primer under an adhesive is a soft layer in the bond line. Follow the stated drying time, and longer in cold or humid conditions.
8. Re-test after priming.Wipe it again. If it still chalks, apply a second coat or accept that the surface needs cutting back further. A primer that has not stabilised the surface has not solved the problem.
9. Add a mechanical fixing where the surface cannot be trusted.On an old lath-and-plaster wall, a soft render or a wall you cannot get sound, the honest answer is a screw into something structural rather than a better primer.

Tips

• The tape test is quick and decisive: press a strip of tape onto the surface, pull it off, and look at the adhesive. Dust on the tape means dust under your adhesive.
• Bare plaster and plasterboard are extremely absorbent and will suck the water out of a water-based grab adhesive before it can cure. Priming there is about suction, not dust.
• Between two non-porous surfaces, a water-based or solvent-based adhesive has nowhere for its carrier to go and can stay soft indefinitely. That is a product-choice problem, not a priming one — use a chemically curing adhesive instead.
• On external render and masonry, use a masonry stabilising solution rather than PVA. PVA outdoors softens with water and fails.
• Old distemper cannot be reliably stabilised and should be washed off. Anything stuck to distemper comes off with it.

Common mistakes

• PVA-ing a wall that will get wet — PVA re-emulsifies with water. Behind tiles, in a shower, or on an external wall, it becomes a slip plane and the whole assembly comes off.
• Priming over loose material — A primer binds dust; it does not reattach a layer that has already parted from the wall. The loose material has to come off first.
• Sealing a damp wall — The moisture is trapped and the failure that follows is worse and larger than the original. Deal with the source first.
• Assuming the adhesive will cope — No adhesive is stronger than the surface it is stuck to. This is the most common reason a correctly chosen product ends up on the floor with a disc of paint or plaster attached.

Questions people ask