How To Glue Rubber to Fibreglass

What is the best product for bonding a rubber sealing strip to fibreglass?

Selleys names two options for this: Roof & Gutter Silicone, which forms a strong durable bond and seal without damaging the surfaces it attaches to, or Selleys 650FC hybrid polymer adhesive sealant.

Before you choose, there is a limitation on the silicone that applies directly to the rubber half of this job, and it is not on the front of the pack. Read the next section before you buy.

The rubber caveat you need to know

The Roof & Gutter Silicone technical data sheet carries this limitation: avoid contact with materials containing high levels of plasticisers, such as rubber and bituminous substrates. Selleys' technical guidance explains why — components in some rubber products can migrate into the sealant, causing staining or reduced performance.

That does not make silicone the wrong answer. Rubber compounds vary enormously, and plenty of sealing strips carry little free plasticiser. What it does mean is that a pre-test is not optional here. Bond a short offcut of your strip to a scrap of the same fibreglass, leave it several days to cure through, then try to peel it and look at the sealant for softening or staining. Selleys' own recommendation on this product is that a sample be applied to the intended area first to confirm appearance, finish and performance.

The same TDS also rules out polyethylene, polypropylene and PTFE entirely. If the "rubber" strip turns out to be a polyolefin extrusion, silicone will not bond it at all.

650 FC is worth considering for the same reason. Its technical data sheet describes very broad primerless adhesion, and while it also recommends a pre-test where adhesion is uncertain, it does not carry the plasticiser limitation the silicone does.

The two products side by side

Property	Roof & Gutter Silicone	Selleys 650 FC
Technology	100% silicone, neutral (moisture) cure	Silane modified polymer
Skin time	5–10 minutes	20 minutes
Tooling time	5 minutes	—
Tack-free time	30 minutes	—
Cure rate	2 mm / 24 hrs	2.5 mm / 24 h
Joint movement	±25%	±20%
Elongation at break	330% (ASTM D412)	>500% (ASTM D412)
Shore A hardness	32	40 ± 5
Application temperature	+5 °C to +30 °C	5–35 °C
Service temperature	−60 °C to +150 °C	−40 °C to +70 °C, brief points at 100 °C
Paintable	No	Yes, once cured

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

The 5-minute tooling window on the silicone is the practical constraint. Have the strip cut, dry-fitted and ready before you open the cartridge, because once the skin starts to form at 5–10 minutes, further tooling tears the surface rather than smoothing it. 650 FC gives you a more relaxed 20 minutes before it skins.

The 2 mm and 2.5 mm per 24 hours cure rates tell you how long the strip needs to stay supported. A 4 mm bed of sealant under a strip is two days from cured, not two hours — and both products need atmospheric moisture to cure, so on two non-porous surfaces like rubber and fibreglass, expect the slower end of the range.

Preparation on fibreglass

Fibreglass carries mould release agent from the moulding process, and it is the most common cause of a failed bond on a new or recently repaired panel.

1. Ensure all surfaces are clean, dry, sound, and free of dust, oil, rust and other contaminants.
2. Clean the non-porous surfaces with a clean cloth dampened with isopropyl alcohol or white spirit, wiping in one direction only, then immediately wipe with a clean, dry cloth. Wiping back and forth just redistributes the contamination.
3. Remove all existing adhesive or sealant if you are replacing an old strip.
4. Cut the cartridge tip, screw on the nozzle and trim it to the bead size you want. A high-thrust-ratio caulking gun makes high-viscosity products much easier to control.
5. Apply a continuous bead, press the strip home, and tool it with a clean spatula dipped in soapy water before the skin forms.
6. Support the strip and leave it undisturbed while it cures.

If the fibreglass is a marine gelcoat surface, Selleys Marine Flex is worth knowing about: it needs no primer on fibreglass, but its instructions call for the gelcoat to be lightly sanded and wiped with mineral turpentine to remove all traces of oil and mould release before application. That is a sound approach to preparing any gelcoat surface.

Limitations

Roof & Gutter Silicone: do not apply if ambient or substrate temperature is below +5 °C or above +40 °C; not for continuous submersion or below-waterline use; not for structural glazing; not paintable (allow paint 7 days to dry before sealing over it); needs atmospheric moisture to cure, so curing slows on non-porous substrates or in low humidity; avoid contact with high-plasticiser materials such as rubber and bituminous substrates; not for use on polyethylene, polypropylene or PTFE.

650 FC: do not apply below 5 °C; requires atmospheric moisture to cure; not recommended for continuous submersion or below water line use; do not apply in the presence of curing silicone or polyurethane sealants; avoid contact with solvent cleaners during cure; not for structural glazing; may affect drying of alkyd/oil-based paints; white may yellow slightly with age and sun exposure. Classified as hazardous — wear gloves and safety glasses.

Common questions

Will the silicone damage the fibreglass?

Selleys' answer for this application is that it forms a strong durable bond and seal without damaging any of the surfaces it may attach to. Neutral cure silicone does not release acetic acid as it sets, which is what makes it gentler on sensitive substrates than an acetic cure product.

So why the pre-test?

Because of the rubber, not the fibreglass. Plasticisers migrating out of some rubbers can stain the sealant or reduce its performance, and there is no way to tell from looking at a strip whether yours will.

How long before the strip is properly held?

Tack-free at about 30 minutes for the silicone, but curing through at 2 mm per 24 hours. Support the strip until the bead has cured through its full thickness.

Which one for a joint that moves?

Either — the silicone offers ±25% joint movement and 330% elongation at break, 650 FC ±20% and greater than 500% elongation. The silicone has the wider service temperature range (−60 °C to +150 °C); 650 FC has the higher hardness and is paintable.

Can I use it underwater or on a hull below the waterline?

Neither product is recommended for continuous submersion or below-waterline use. Marine Flex is the Selleys product formulated for above and below waterline work.

Can I paint the bead?

650 FC yes, once cured. Roof & Gutter Silicone no — it is not paintable.

How do I clean up?

Turps on tools and equipment before the product cures. Cured sealant comes off mechanically or with Selleys Sealant Remover — though note Sealant Remover is not suitable for rubber, EPDM or polycarbonate, so keep it off the strip itself.