Best Product To Seal a Fibreglass Swimming Pool

What is the best product to seal beneath a coping tile and the top of a fibreglass swimming pool?

Use Selleys 655FC. It is a premium 100% clear flexible adhesive and sealant powered by Selleys advanced hybrid technology. It offers very broad primerless adhesion with a crystal clear finish, and it is built to withstand outdoor conditions. Once cured it provides superior strength with an instant grab, and — the property that matters most on a pool edge — it can be applied to wet or damp surfaces with excellent adhesion.

That last point is the reason it beats most alternatives here. A pool coping joint is rarely genuinely dry. Splash, condensation and seepage from the surrounding paving mean that a sealant requiring a bone-dry substrate is being asked for something the site cannot deliver. 655FC does not need it.

The limitation to be clear about before you start

655FC is not for permanent water immersion, and it should not be exposed to continuous water before it has cured.

That is compatible with this job as described — the joint beneath a coping tile at the top of the shell sits above the water line — but it defines the boundary. Do not use it to seal anything below the water line, and do not fill the pool so the new bead is submerged while it is still curing. Let it cure first.

The published figures

Technology	Moisture curing silyl-modified polymer (SMP)
Colour	Clear
Packaging	290mL cartridge
Density	Approximately 1.06 g/mL
Set time	20 minutes (23°C, 50% RH)
Tooling time	Approximately 25 minutes
Skin time	45 to 55 minutes
Cure rate	Approximately 3mm per 24 hours
Full cure	24 hours for full strength
Application temperature	5 to 35°C
Service temperature	0 to 70°C
Tensile strength	1 ± 0.1 MPa (DIN 53504)
Shore A hardness	30 ± 5
Elongation at break	Approximately 130% (DIN 53504)
Slump	0mm (non-sagging)
VOC content	Approximately 35 g/L

Indicative data only, not intended for the preparation of specifications.

What those numbers mean at the pool edge

Non-yellowing. A clear sealant in full sun is the hardest test a clear product faces. 655FC is specified as non-yellowing, which is what keeps a joint under a coping tile looking clean rather than amber after a couple of summers.

25 minute tooling time. Generous, and useful on a long run around a pool. You can lay several metres and go back to tool them properly, rather than working in short sections.

3mm per 24 hours. A bead 6mm deep is still curing on day two. Full strength arrives at 24 hours for a normal bead, but plan the refill around the depth you have applied, and keep the joint out of standing water until it is done.

Instant grab, 0mm slump. On a vertical face under a coping edge, the bead stays where you put it rather than sagging out of the joint while it sets.

How to seal the joint

1. Remove any old, failed sealant completely. Fresh sealant bonds to the substrate, not to the remains of the previous bead.
2. Clean both faces of wax, grease, dirt and loose material. Damp is acceptable; contaminated is not.
3. Cut the nozzle to the width of the joint and fit it to the 290mL cartridge. Load a caulking gun.
4. Extrude the sealant into the joint, forcing it in so it makes full contact with both the coping and the fibreglass rather than bridging across the top.
5. Tool the bead within about 25 minutes, leaving a smooth face that sheds water.
6. Clean up uncured product with mineral turps. Cured product needs scraping, sanding, or a silicone remover.
7. Keep the joint out of continuous water until it has cured.

Limitations

• Not for permanent water immersion, or continuous water exposure before curing.
• Not suitable for polyethylene, polypropylene or Teflon.
• Not recommended on bituminous substrates.
• Cannot be painted with solvent-based paints unless a water-based paint is applied first. It is paintable with water-based paints after curing.
• Joint width for vertical or overhead joints is limited to 20mm.
• Apply between 5 and 35°C; extrusion and cure are better above 5°C.
• Store below 30°C and use within 6 months of opening.

If the joint is masonry rather than fibreglass

Where the gap is under coping pavers against a brick wall rather than against a fibreglass shell, Brick & Concrete Silicone is the product Selleys nominates — see filling the gap between a brick wall and a swimming pool. Note that it too is not suitable for below waterline applications.

Safety: two products on this page, two very different classifications

655FC carries the signal word Warning, with eye irritation Category 2A (H319, causes serious eye irritation) and skin sensitisation Category 1 (H317, may cause an allergic skin reaction). It is not Dangerous Goods by land, sea or air.

H317 is the one that bears on the method above. Sensitisation is not irritation — it is permanent once acquired, triggered thereafter by far smaller exposures, and built by repeated casual contact. A full pool perimeter is a long run, and the generous 25-minute tooling window is exactly what tempts people to smooth it with a bare wet finger. That is a great deal of skin contact in one afternoon. Tool with a spatula or a gloved finger. On H319: you are working at ground level looking down into the joint, and a cartridge under gun pressure spits when the trigger releases — safety glasses.

The product with the serious classification is the one in the last answer above. Selleys Sealant Remover, used to strip the old bead, is classified Danger, Poison Schedule S5, and is over 60% petroleum naphtha: H304 may be fatal if swallowed and enters airways, H226 flammable liquid and vapour (Dangerous Goods Class 3, UN 1133, Packing Group II), H336 may cause drowsiness or dizziness, and H315 causes skin irritation.

This is the step that scales badly. A layer at least 5mm thick, left at least ten minutes, often applied twice, around an entire coping run is the largest quantity of solvent paste any job of this kind asks for. Working outdoors takes most of the sting out of H336, but it does nothing about H226. The ignition source most likely to be within a couple of metres of pool coping is the plant: the pump motor and, if there is one, the gas heater. Do not strip a bead alongside a running pump or an operating heater, and no smoking.

H304 around a pool is a supervision point. An open tub of light beige paste at ground level, in the one part of a property where children congregate, is exactly the wrong place to leave it. Lid on between applications, keep it out of reach, and never decant it into another container. If any is swallowed, do not induce vomiting — rinse the mouth, give a glass of water and seek medical advice immediately. The mineral turps for clean-up is flammable in its own right.

The lower-hazard route is already in step 1: remove the old sealant completely by cutting and scraping. The remover is designed for a residual film under 5mm, so the more comes off mechanically, the less solvent goes anywhere near the water.

Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions

Can I apply it if the surface is damp?

Yes. 655FC can be applied to wet or damp surfaces with excellent adhesion, which is unusual and genuinely useful here.

Can I use it below the water line?

No. It is not for permanent water immersion.

Will it stay clear?

It is specified as non-yellowing, and provides a crystal clear finish.

Does it need a primer?

No — broad primerless adhesion is one of its stated features.

How long before the pool can be refilled?

Allow it to cure first. Full strength is reached at 24 hours for a normal bead, and the cure runs at approximately 3mm per 24 hours, so a deep bead takes longer.

How long do I have to tool it?

Approximately 25 minutes, with skin forming at 45 to 55 minutes.

Can I paint it?

With water-based paints, once cured. Solvent-based paints need a water-based paint first.

What size does it come in?

A 290mL clear cartridge.

How wide a joint can it fill?

Up to 20mm wide for vertical or overhead joints.

How do I remove the old sealant first?

Cut and scrape the bulk of it, then soften the residue with Selleys Sealant Remover — spread at least 5mm thick, left at least 10 minutes, then scraped off and cleaned up with mineral turps.