How To Install Cornices
How do you install cornices with construction adhesive?
Cornices do a lot for the character of a room, and fixing them with a construction adhesive is far quicker than trying to mechanically fasten a moulding into the junction of a wall and ceiling. The method is short: clean both surfaces, cut the nozzle to about 5 mm, run a bead along the top and bottom of the back of the cornice, and press it into position quickly. The product this guide uses is Selleys Liquid Nails Fast Grab, a water-based construction adhesive with strong initial hold, which is what makes it suitable for a job where you are holding a length of moulding overhead.
The single most important number here is the open time. Liquid Nails Fast Grab's technical data sheet says to bring the surfaces together as soon as possible — within the first 5 minutes. That governs how you plan the job: cut and dry-fit every length before you apply any adhesive, and glue one piece at a time.
Technical specification
| Technology | Water-based |
| Colour | Beige |
| Instant grab | Up to 100 kg/m² (timber to timber) |
| Bond strength in tensile shear | >1.5 MPa (AS 2329-1999E) |
| Strength after 7 days | Up to 500 tonnes/m² (timber to timber) |
| Open time | Bring surfaces together within the first 5 minutes |
| Before removing temporary fasteners | Minimum 24 hours |
| Cure time | 72 hours |
| Paintable | After 12 hours |
| Gap bridging | Up to 9 mm |
| Packaging | 420 g cartridge, 130 g tube |
Indicative data only, not intended for the preparation of specifications.
What "instant grab" actually buys you
Up to 100 kg/m² of instant grab is what lets a cornice stay where you put it instead of sliding down the wall while you fetch a prop. It is not the same as strength. The bond keeps building for days: minimum 24 hours before you take temporary fasteners off, 72 hours to cure, and up to 500 tonnes per square metre on a timber-to-timber bond after 7 days.
The practical consequence is that the first day is the fragile one. Do not lean a ladder against a freshly fixed cornice, and do not paint it for at least 12 hours.
The 9 mm gap-bridging figure is the other useful number on a cornice job, because wall and ceiling junctions are rarely straight. The adhesive can fill and hold across a gap up to that width, which means minor waviness in the plaster does not have to be packed out. Anything wider is a packing problem, not an adhesive one.
What you need
- Selleys Liquid Nails Fast Grab
- A caulking gun
- Cornices cut to size
- Mechanical fasteners
- A damp cloth and water for clean-up
How to install the cornices
- Prepare the surfaces. Make sure both surfaces are clean, dry and free of oil, grease and dust. The adhesive is water-based and gives its best results where one surface is porous — plaster and timber both qualify, so a dusty surface is the main thing standing between you and a good bond.
- Cut and dry-fit first. Have every length cut to size and offered up before you open the cartridge. With a 5-minute window there is no time to trim a mitre with adhesive already on the back.
- Prepare the cartridge. Cut the cartridge tip and cut the nozzle to about 5 mm.
- Apply the adhesive. Use the caulking gun to apply a bead of Liquid Nails Fast Grab to the top and bottom of the back of the cornice. On broad flat surfaces the data sheet calls for beads every 40 cm; on a cornice, the two edges are what make contact with the wall and ceiling.
- Fix it in place quickly. Bring the surfaces together as soon as possible — within the first 5 minutes. Place the cornice firmly into position and press it against the wall.
- Support it. Use mechanical fasteners with the adhesive in structural, critical or high-stress applications. Supports are also recommended for heavy items and overhead applications, which a cornice is.
- Leave it alone. Allow a minimum of 24 hours before removing any temporary fasteners or clamping. Full cure is 72 hours.
- Clean up. Fast clean-up with water while the adhesive is still wet. Cured product has to be scraped or sanded off.
Limitations — where Liquid Nails Fast Grab should not be used
The technical data sheet is specific, and several of these come up on interior fit-out work.
- Not for use on polyethylene, polypropylene or PTFE. Pre-test other plastics and rubbers to determine compatibility.
- Not for mirrors. Use Liquid Nails Mirror & Metal instead.
- Not for metals heated by direct sunlight — bonds weaken at higher temperatures.
- Do not use as the sole bonding agent for structural bonding. Use mechanical fasteners in high-stress applications.
- Avoid use in tightly clamped joints such as cabinet making or joinery.
- Interior and sheltered exterior applications protected from moisture only.
- Expect longer cure times in cold or humid weather and on less porous surfaces. Best results are obtained where one surface is porous.
That last point is worth a moment on a cornice job. Plaster and plasterboard are porous, so a standard plaster cornice onto a plasterboard wall is close to ideal for this adhesive. A polystyrene or plastic cornice is a different proposition — the data sheet does list polystyrene foam among suitable materials, but plastics should be pre-tested, and polyethylene, polypropylene and PTFE are excluded outright.
Painting and finishing
Liquid Nails Fast Grab is paintable after 12 hours. If you are filling the joint line between the cornice and the wall afterwards, Selleys No More Gaps Interior Multipurpose is the flexible filler for that kind of moving joint, with a maximum recommended gap of 10 mm wide by 5 mm deep for good paint crack resistance.
Common questions about installing cornices
How long do I have to position the cornice?
Bring the surfaces together as soon as possible, within the first 5 minutes of applying the adhesive.
How long before I can take the props or tape off?
A minimum of 24 hours. Full cure is 72 hours, and longer in cold or humid weather.
Do I still need mechanical fasteners?
Selleys says Liquid Nails Fast Grab should be used with mechanical fasteners in structural, critical or high-stress applications, and recommends supports for heavy items and overhead work. It should never be the sole bonding agent for structural bonding.
How wide should I cut the nozzle?
About 5 mm for this job.
How much of a gap can the adhesive bridge?
Up to 9 mm.
When can I paint the cornice?
After 12 hours.
How do I clean off the excess?
Water, while it is still wet. Cured adhesive has to be scraped or sanded off.
Can I use it in a bathroom or outside?
It is rated for interior and sheltered exterior applications protected from moisture only. It is not for exposed exterior work.
Will it stick to a plastic or polystyrene cornice?
Polystyrene foam is listed among suitable materials, but plastics and rubbers should be pre-tested, and it will not bond polyethylene, polypropylene or PTFE.
What if the ceiling line is not straight?
The adhesive bridges gaps up to 9 mm, which covers most minor waviness. Larger discrepancies need packing rather than more adhesive.
How strong is the finished bond?
Greater than 1.5 MPa in tensile shear tested to AS 2329-1999E, building to as much as 500 tonnes/m² on a timber-to-timber bond after 7 days.
Is it low odour?
It is a water-based formulation, which the data sheet describes as low odour and therefore suited to indoor applications, with water clean-up.
What is the purpose of cornices in home improvement, according to Selleys?
Cornices are decorative architectural elements commonly used in interior building construction to add aesthetic appeal and define the transition between walls and ceilings.
What surface preparation is needed before installing cornices?
Before beginning installation, all surfaces should be clean, dry, and free of dust, grease, old sealant, or flaking paint, as proper surface preparation is critical for achieving strong, lasting results.
What adhesive does Selleys recommend for cornice installation, and what are its properties?
Pro Trade Construction Adhesive Fast Grip is a high-strength construction adhesive suitable for cornices. It provides strong initial holding power and flexible bonding, making it ideal for vertical applications. It is water-based, easy to clean up with water, and can be painted after 12 hours.
What is an alternative adhesive option for cornice installation, and what are its features?
Selleys Liquid Nails Fast Grab is another option for cornice installation. It offers up to 100 kg/m² instant grab and bridges gaps up to 9mm, making it useful for uneven surfaces. It is also water-based, low odour, and suitable for interior use.
How should adhesive be applied when installing a cornice?
Cut the cartridge tip and fit the nozzle appropriately, then apply a continuous bead of adhesive along the length where the cornice will be attached. The cornice should be brought into position within the first 5 minutes of application.
What should be done for heavy cornices or overhead applications?
For heavy items or overhead applications, use sufficient nails, screws, or clamping to hold the load and achieve contact over the total bond area.
How long should adhesive set before removing temporary fasteners or clamps?
The adhesive should be allowed to set for a minimum of 24 hours before removing any temporary fasteners or clamping.
What product is recommended for filling gaps after cornice installation?
Selleys No More Gaps Interior Fast Dry is ideal for filling gaps that appear between the cornice and adjacent surfaces after installation.