Perlfix Adhesive Guide — Coving & Plasterboard
Knauf Perlfix is a gypsum-based adhesive widely used in the UK for bonding plasterboard, coving, cornices and decorative mouldings to masonry and concrete surfaces. It's the go-to adhesive for dot-and-dab plasterboard installations and is equally effective for fixing polystyrene and plaster coving. This guide covers coverage rates, mixing, application techniques and how Perlfix compares with alternatives.
What Is Perlfix Adhesive?
Knauf Perlfix is a factory-mixed, gypsum-based bonding compound supplied as a dry powder in 25 kg bags. It's designed for internal use only and bonds plasterboard, insulated plasterboard (thermal laminate), coving and decorative plaster elements directly to solid backgrounds such as brick, block, concrete and aerated block.
Unlike general-purpose grab adhesives, Perlfix offers a long open time for positioning, strong permanent adhesion and easy clean-up before setting. It is non-toxic and has no solvent odour, making it suitable for enclosed spaces.
Coverage Rates
| Application | Coverage | Notes |
|---|---|---|
| Dot-and-dab (plasterboard) | 2.5–3.5 kg/m² | Depends on wall flatness; more adhesive needed on uneven surfaces |
| Coving (polystyrene or plaster) | 0.5–1 kg per linear metre | Varies with coving profile size and wall condition |
| Decorative mouldings / corbels | Approx. 3–5 kg/m² | Heavier items may need temporary support while setting |
One 25 kg bag covers approximately 7–10 m² for dot-and-dab or 25–50 linear metres of coving.
Perlfix vs Alternatives — Comparison
| Feature | Knauf Perlfix | Gyproc Dri-Wall | General Grab Adhesive |
|---|---|---|---|
| Type | Gypsum-based powder | Gypsum-based powder | Solvent or water-based paste |
| Pot life | ~30 minutes | ~30 minutes | Ready to use (tube) |
| Initial set | ~1 hour | ~1.5 hours | 10–30 minutes |
| Full cure | 24 hours | 24 hours | 24–48 hours |
| Best for | Dot-and-dab, coving, mouldings | Dot-and-dab plasterboard | Small repairs, lightweight trims |
| Adjustability | Good — can reposition before set | Good — can reposition before set | Limited once applied |
| Approx. cost (25 kg) | £8–£12 | £8–£12 | £3–£6 per tube (310 ml) |
Perlfix and Gyproc Dri-Wall are functionally interchangeable for most applications. Choice usually comes down to availability and trade preference. General grab adhesive is not suitable for full dot-and-dab installations.
Application Method for Coving
When fixing coving with Perlfix, the butter method gives the strongest bond:
- Mark a guideline on the wall and ceiling at the correct offset for your coving profile.
- Dampen the wall and ceiling surfaces with a sponge to improve adhesion.
- Apply a continuous bead of mixed Perlfix (roughly 10 mm thick) to both the wall-facing and ceiling-facing edges of the coving length.
- Press the coving firmly into position along your guidelines, sliding gently to spread the adhesive.
- Remove any excess adhesive that squeezes out with a damp sponge or filling knife before it sets.
- For internal corners, mitre cut at 45° or use pre-formed corner pieces. Apply Perlfix to the mitre joint as well.
- Support heavy plaster coving with temporary pins or masking tape until the initial set (~1 hour).
For lightweight polystyrene coving, the adhesive alone provides sufficient hold immediately. Plaster coving over 127 mm (5″) may need temporary support.
Application for Dot-and-Dab
Dot-and-dab (also called direct bonding) is the standard method for fixing plasterboard to masonry walls:
- Apply a continuous ribbon of Perlfix around the perimeter of the board area, especially along the floor line for fire stopping.
- Apply dabs approximately 50–75 mm wide and 250 mm tall at 600 mm horizontal centres and three rows vertically (top, middle, bottom).
- Offer the plasterboard to the wall and press firmly, using a straight edge to check alignment.
- Tap the board with a rubber mallet or padded timber to achieve a true, plumb surface.
- Leave a 10 mm gap at the floor for expansion. Adhesive dabs should be 10–25 mm thick when compressed.
For thermal laminate boards (insulated plasterboard), increase the dab size and number to support the additional weight. Check the board manufacturer's fixing guide.
Mixing Instructions
Mix Perlfix by adding the powder to clean, cold water in a clean bucket:
- Ratio: approximately 0.5–0.6 litres of water per 1 kg of powder (roughly 12–15 litres per 25 kg bag).
- Add the powder to the water (not water to powder) to avoid lumps.
- Mix with a paddle mixer or by hand until you have a thick, lump-free paste — similar to stiff porridge.
- Pot life: approximately 30 minutes. Only mix what you can use in this time.
- Do not add water to re-temper adhesive that has started to stiffen — discard it and mix a fresh batch.
Setting Time
- Initial set: approximately 1 hour at 20°C. The adhesive will hold the board or coving in position but is not yet fully cured.
- Full cure: 24 hours. Do not apply loads, tape joints or decorate until fully cured.
- Cold conditions (below 5°C) significantly slow setting. Ensure the working area is at least 5°C and adequately ventilated.
Storage and Shelf Life
- Store bags in a dry, sheltered area off the ground on pallets or boards.
- Shelf life is approximately 6 months from date of manufacture when stored correctly.
- Discard any powder that has hardened, become lumpy or has been exposed to moisture — it will not mix properly and bond strength will be compromised.
- Once opened, fold the bag tightly closed and use within 2–4 weeks.
Calculate Your Adhesive Needs
- Coving Calculator — work out coving lengths and adhesive needed for any room
- Dot and Dab Calculator — calculate adhesive for dot-and-dab plasterboard fixing
- Dry Lining Calculator — materials for dry lining walls including boards, tape and jointing compound
Coverage rates and setting times are approximate and based on Knauf published data for standard conditions (20°C, moderate humidity). Always check the product data sheet for the batch you are using. Perlfix is for internal use only and should not be used in wet areas or below the damp-proof course.