What is Selleys Flexiseal FC, and where should you use it?

Selleys Flexiseal FC is a non-slumping polyurethane adhesive and sealant that does two jobs from one cartridge: it bonds common building materials, and it seals construction joints in both vertical and horizontal applications. It is weatherproof, suitable indoors and outdoors, and carries ±25% joint movement capability, certified to ISO 11600/F/25HM.

The thing to settle before you buy it is whether polyurethane is the right chemistry for your job. Flexiseal FC is classified as hazardous and contains isocyanates — gloves and safety glasses are required. It also cures with atmospheric moisture, will not adhere to PE, PP or PTFE, and is explicitly ruled out for structural and general glazing. Within those boundaries it is a strong, highly elastic construction sealant; outside them it is the wrong product.

Technical specification

Property	Typical result
Technology	Polyurethane
Colour	Grey, black, white
Density	1.29
Application temperature	5°C to 35°C
Tack free time (23°C, 50% r.h.)	55 to 65 minutes
Curing (23°C, 50% r.h.)	2.5mm per 24 hours
Shore A hardness (DIN 53505)	40 to 45
Joint movement capability	±25%
Elongation at break (DIN 53504)	Greater than 550%
Elastic modulus at 100% (DIN 52455)	0.8 N/mm²
Tensile strength (DIN 53504)	Greater than 1.8 N/mm²
Service temperature	-40°C to +90°C, with brief points at 120°C
Shelf life	12 months
Packaging	600mL sausage, in a range of colours

Contact Selleys before using this data for the setting of specifications. Note that the product page also lists a 310mL cartridge alongside the 600mL sausage, with availability varying by colour; the technical data sheet describes the 600mL sausage only.

What the key numbers mean in practice

The tack free time of 55 to 65 minutes at 23°C and 50% relative humidity is generous by sealant standards — close to an hour of working time in average conditions. That is what makes Flexiseal FC practical for long external joints where you cannot realistically tool a bead within ten minutes. Remember the conditions attached to the figure: hotter, more humid weather shortens it.

The cure rate of 2.5mm per 24 hours describes how far the cure front travels into the bead each day, not how long the joint takes overall. Because the product requires atmospheric moisture to cure, a deep joint takes proportionally longer, and a joint sealed off from the air behaves differently again. The product page puts the practical version of this simply: the initial bond takes around 24 hours, and full strength develops over days.

±25% movement capability, with elongation at break above 550%, is what qualifies it for external wall and cladding joints that expand and contract with the weather. The certification to ISO 11600/F/25HM records that movement class formally. But the number only holds if the bead is free to move — see the joint geometry below.

A service temperature of -40°C to +90°C, with brief excursions to 120°C, covers roofing and external cladding in Australian conditions comfortably.

Joint sizing and coverage

The minimum joint width and depth is 6mm. For joint widths up to 12mm, use a 1:1 width-to-depth ratio; for widths greater than 12mm, use 2:1 — the stated optimum for joint sealing. Always install a suitable diameter backing rod (polyethylene or polyurethane foam) to control the depth and let the sealant move freely, and remember that the sealant must adhere to the sides of the joint only, not to the back, so it is free to stretch. Wider joints accommodate more total movement than narrow ones.

Published coverage per sausage:

Joint width (mm)	Joint depth (mm)	Litres per lineal metre
6	6	0.036
8	8	0.064
10	10	0.100
12	12	0.140
14	7	0.098
16	8	0.128
18	9	0.162
20	10	0.200
22	11	0.242
24	12	0.288
26	13	0.338
28	14	0.392
30	15	0.450
40	20	0.800

Where to use it

Flexiseal FC is intended for bonding common building materials and sealing construction joints, including:

• External wall and cladding joints
• Weatherproofing joints between brickwork, masonry, wood, concrete or metal frames
• Joints in walls, balconies, and around window or door frames
• Joints in bridge and balcony parapets
• Retaining walls
• Roofing
• Acoustic rated walls — it will not degrade acoustic walls that meet the BCA requirements

As an adhesive, it is used for indoor and outdoor bonding of window sills, thresholds, stair steps, skirting boards, base boards, crash protection boards, covering boards and prefabricated elements.

Adhesion is described as excellent on concrete, masonry, brick, ceramic tile, metals, soft wood, hardwood and some plastics, with no shrinkage during curing and excellent resistance to ageing and weathering.

Priming and pre-testing

Flexiseal FC generally has good adhesion without priming to the common building materials listed. In some instances a primer may be required, and a pre-test to confirm suitable adhesion is recommended. Given the explicit exclusion of PE, PP and PTFE and the instruction to test other plastics first, a trial bond on an offcut is the sensible step whenever plastics are involved.

How to apply it

As a sealant

1. Prepare the surface. It must be clean, dry and free from oil, grease, release agents, dust and loose material.
2. Install backing rod to the correct depth and apply masking tape as required.
3. Cut the end of the sausage and insert it into a Selleys sausage gun.
4. Extrude the sealant into the joint so it is in full contact with the sides. Avoid overlapping beads, which traps air.
5. Smooth with a spatula, forcing the sealant into the joint for better contact with the sides.
6. Allow it to cure before painting. Painting with water-based paints is possible after curing; if you are using solvent-based paints, prime first with a water-based primer.

The manufacturer notes that these instructions are a guide, and that the installer should apply all Australian Standards, BCA/NZBC requirements and best practice when installing.

As an adhesive

1. Clean both surfaces so they are dry and free from oil, grease, release agents, dust and loose material.
2. Apply Flexiseal FC in spots or beads on the prepared surfaces.
3. Press the parts firmly together.
4. Use suitable fixing to hold the assembly until the adhesive has cured. The product page adds that excess should be removed immediately.

Limitations — where Flexiseal FC should not be used

These are the manufacturer's stated limits:

• Do not apply if air or substrate surface temperature is below 5°C.
• It requires atmospheric moisture to cure.
• Not recommended for continuous submersion or below water line use.
• Do not apply in the presence of curing silicone sealants — the cure may be affected.
• Avoid contact with alcohol and other solvent cleaners during cure, which will affect the cure.
• Application to substrates with high moisture can cause bubbling within the sealant.
• Do not use for structural or general glazing.
• Avoid air entrapment when applying it as a sealant.
• May affect the drying of alkyd and oil-based paints.
• White sealant may yellow slightly with age and exposure to the sun.
• Do not store the product above 30°C.

The product page adds that it will not adhere to PE, PP or PTFE and that other plastics should be tested first, that some colours may discolour after UV exposure, and that it is not intended for structural load. To maximise durability in outdoor applications, paint with a suitable exterior paint.

Two of these do more damage than the rest. Applying over a high-moisture substrate causes bubbling inside the bead, which looks fine on day one and fails later; and a joint sealed while a nearby silicone is still curing may simply not cure properly at all.

Safety

Flexiseal FC is classified as HAZARDOUS and contains isocyanates. Wear gloves and safety glasses when using it. It is not classified as Dangerous Goods. Refer to the Safety Data Sheet for detailed information.

Clean up and storage

Spills and tools clean up with mineral turps before the product cures. Cured product can only be removed mechanically.

Store unopened sausages below 30°C in a cool dry place away from direct sunlight; unopened, they keep for 12 months from the date of manufacture. Opened sausages may skin and form a lump near the opening — discard the lump and skin, and use the rest.

Common questions about Selleys Flexiseal FC

Can I use it as both a glue and a sealant?

Yes. It is a multipurpose sealant and elastic adhesive, with separate application instructions for each use.

How long before it is tack free?

55 to 65 minutes at 23°C and 50% relative humidity.

How long until it is fully cured?

It cures at 2.5mm per 24 hours, so the total depends on bead depth. The product page describes the initial bond as taking about 24 hours, with full strength developing over days.

Can I paint over it?

Yes, with water-based paints, after it has cured. For solvent-based paints, prime first with a water-based primer. Use a suitable exterior paint outdoors for maximum durability.

Will it stick to plastic?

Not to PE, PP or PTFE. It adheres to some plastics, but those should be tested first, and a primer may be required.

Can I use it for glazing?

No. The TDS excludes both structural and general glazing.

Can I use it below the water line or on a submerged joint?

No. It is not recommended for continuous submersion or below water line use.

What is the coldest temperature I can apply it at?

Do not apply if the air or substrate temperature is below 5°C. The stated application temperature range is 5°C to 35°C.

Will the white go yellow?

The white sealant may yellow slightly with age and exposure to the sun, and the product page notes some colours may discolour under UV.

Is it safe to use without protective equipment?

No. It is classified as hazardous and contains isocyanates. Wear gloves and safety glasses, and refer to the Safety Data Sheet.

How much will one sausage do?

A 6mm x 6mm joint uses 0.036 litres per lineal metre; a 20mm x 10mm joint uses 0.200 litres per lineal metre. Use the coverage table above to size your order.

What else should I look at for construction joints?

Browse the construction silicone range, including Flexiseal Multipurpose Sealant and Brick & Concrete Silicone, or the full multi-purpose sealant range.