How To Seal a Toilet
Which product should you use at the base of a toilet to bed it down?
A silicone sealant such as Selleys No Mould is ideal for bedding a toilet pan down. Once the pan is positioned and sealed in place, you must leave it undisturbed for at least 72 hours.
That 72-hour figure is longer than the cure time quoted for an ordinary bathroom bead, and the reason is worth understanding — it is the difference between a thin bead in a joint and a thick bed under a heavy fixture.
Why bedding a pan takes longer than sealing a joint
No Mould cures at a rate of 2mm per 24 hours. That is the speed at which the reaction with atmospheric moisture works inward from the exposed surface.
A bead in a shower corner is thin and has two exposed faces, so it cures through quickly. A bed of silicone under a toilet pan is thicker, and it is almost entirely enclosed — the only air reaching it is at the perimeter. The material in the middle of that bed is the last to cure, and it is precisely the material carrying the weight of the pan.
Move the pan before that has happened and you shear the uncured silicone. The bed never re-forms, and what you are left with is a pan sitting on a partially bonded, partially smeared layer that will rock and eventually leak at the floor.
Hence the instruction: positioned, sealed, then left alone for at least 72 hours. Not sat on, not nudged square, not adjusted.
Published figures for No Mould
| Property | Selleys No Mould |
|---|---|
| Technology | Acetic cure silicone sealant, with Speed Seal technology |
| Skin time | 20 minutes |
| Curing rate | 2mm / 24 hours |
| Joint movement capability | ±25% (ISO 9047) |
| Elongation | Approximately 400% (ASTM D412 modified) |
| Waterproof / shower ready | 1 hour after application |
| Full cure (normal bead) | At least 48 hours, undisturbed |
| Packaging | 290mL cartridge; 100mL tube |
| Colours | Transparent, white |
Indicative data only, not intended for the preparation of specifications. Note the two different figures above: the technical data sheet gives at least 48 hours for a normal bead to cure fully, while Selleys' advice for bedding a toilet pan specifies at least 72 hours undisturbed. The longer figure is the one that applies to this job, for the reason set out above.
Why a wet-area silicone suits this joint
A toilet base has to deal with occasional water on the floor, cleaning chemicals, and the humid environment of a bathroom. No Mould is described by Selleys as the product for humid and damp environments such as kitchens and bathrooms where mould is normally present, and it produces a long-lasting waterproof seal that resists lifting.
The ±25% movement capability matters here too. A toilet pan is used constantly and the floor around it takes load. A rigid bedding material would crack; a silicone bed flexes and stays attached.
How to do it
- Check the floor and the underside of the pan are clean and dry. Clean the surfaces where sealant will be applied with turps and let them dry — that is step 2 of No Mould's own directions.
- If you are replacing a pan and there is old, mouldy silicone in the area, spray the space with Selleys Rapid Mould Killer to kill residual mould before you seal, then rinse and dry thoroughly.
- Cut the cartridge tip and cut the nozzle at 45 degrees.
- Apply with steady pressure, making sure the sealant is pushed into contact rather than laid on the surface.
- Position the pan and make your final adjustments now. Once it is down, it stays down.
- Tool the visible perimeter bead with a spatula dipped in detergent water within 10 minutes of application.
- Clean up excess with turps before the product skins.
- Leave the pan undisturbed for at least 72 hours. Keep the toilet out of use for that period.
Plumbing connections, the pan connector and any mechanical fixings are a separate matter and should follow the pan manufacturer's instructions and the relevant plumbing requirements. The sealant beds and seals the pan; it is not a substitute for a correctly made waste connection.
Limitations — where No Mould should not be used
- Not paintable. Complete any painting seven days before applying the sealant.
- Not suitable for food or drinking water contact.
- Not suitable for permanent water immersion or for aquariums.
- Not suitable for metals other than aluminium and stainless steel.
- Not suitable for PE, PP or PTFE plastics. Check what any bearing pads or shims under the pan are made of.
- Keep the bead clean. No Mould's mould resistance is stated on the condition that dirt and soap scum are regularly cleaned off.
Common questions about bedding a toilet pan
Why 72 hours and not 48?
48 hours is the technical data sheet figure for a normal bead reaching full cure. A bed under a pan is thicker and largely enclosed, and silicone cures at 2mm per 24 hours from the exposed face inward, so it needs longer. For this job, use the 72-hour figure.
Can the toilet be used during that time?
No. Using it means sitting on it, which moves the pan and shears the uncured bed. Plan the work around having the toilet out of service.
Should I seal all the way around the base?
Follow the pan manufacturer's and your plumber's guidance on whether to leave a gap at the rear. Some installations deliberately leave a small unsealed section so a leak from the pan connector shows itself on the floor rather than being trapped under the pan.
Which colour should I use?
No Mould is available in transparent and white. White suits most pans and floors; transparent disappears on tile.
What if the floor is not level?
Pack and shim the pan level before you seal, using the pan manufacturer's recommended method. Silicone is a sealant and bedding compound, not a levelling compound.
How do I clean up the excess?
Mineral turpentine, before the product skins. Cured silicone has to be removed with Selleys Sealant Remover or by cutting and scraping.
Can I speed the cure up?
No. These silicones cure by reacting with atmospheric moisture, so warm dry air from a heater or hair dryer does not help and can hinder it. Ventilation at normal humidity is what you want.
Can I use a neutral cure silicone instead?
Selleys Wet Area Silicone is the neutral cure wet-area product, and is the choice where the joint touches natural or engineered stone. On a standard tiled or vinyl bathroom floor, either chemistry is appropriate.
How long before I can seal the joint if I have just removed an old pan?
Once the area is fully clean and dry. Silicone will not bond reliably to a damp surface, and a bathroom floor holds moisture longer than it looks.
Which Selleys product should be used to seal a toilet base to bed it down?
Selleys Wet Area Silicone Sealant is recommended for sealing a toilet to its base, as it is ideal for bathroom applications and provides a superior waterproof seal. Selleys Wet Area Matt Silicone is also suitable, and PRO-TRADE All Purpose Silicone can work as both an adhesive and sealant for bathroom fixtures.
How should I prepare the surface before sealing a toilet to the flange?
Clean the surface with mineral turpentine to remove any dirt, grease, oil, or residue, then allow it to dry completely before applying sealant. If replacing an existing seal, remove any old sealant first.
What is the correct method for applying sealant around a toilet base?
Cut the sealant nozzle at a 45-degree angle to achieve your desired bead width, then apply a continuous bead of sealant around the toilet base where it meets the flange, ensuring full contact with both surfaces. Smooth the sealant with a spatula, forcing the product into the joint for better contact with the sides, and tool the bead within 5-10 minutes of application for a neat finish.
How can I get a neater, cleaner seal around the toilet base?
Apply masking tape to both sides of the joint before applying the sealant, then remove the tape before the sealant skins over, which typically happens 5-10 minutes after application.
How long does the sealant need to cure before it can handle water pressure?
The sealant should be allowed to fully cure for approximately 48 hours before it is subjected to water pressure or movement.
Is the sealant suitable for permanent water immersion?
No, the sealant is not suitable for permanent water immersion, although once fully cured it provides a waterproof seal that prevents water infiltration.
How do I clean up sealant when sealing a toilet to its base?
Uncured sealant can be cleaned with mineral turpentine before it dries. Cured sealant can be removed by scraping, sanding, or using Selleys Sealant Remover.