Which Selleys multi-purpose sealant do you need, and how do you choose?

This is the section of the Selleys sealant range that is not tied to one location. Everything else in sealants is chosen by where the joint is — a shower, a roof, a masonry joint, a fire-rated penetration. The ten products here are chosen by what the joint has to do.

Three questions sort them. Does the joint need to bond as well as seal? Does it need to be invisible? And does it need to be painted? Each of those has a purpose-built answer in this range, and picking on that basis is far more reliable than picking on the word "multipurpose".

What this category covers

The product listing on the live category page is loaded dynamically and could not be captured, so this page is built from the ten products the live sitemap lists under this path. It spans the hybrid-technology adhesive sealants of the 600 series, two clear sealants, a paintable sealant, a neutral cure all-purpose silicone, and a specialist RTV silicone for gaskets and electrical work.

How to choose

If the joint has to carry load as well as exclude water

The 600 series are adhesive sealants — products that bond the two surfaces together as well as sealing between them. Four are listed here, and they separate by how fast they build strength and what they look like:

• Selleys 650FC Fast Cure Adhesive Sealant — fast curing with strong adhesion, for DIY projects and industrial use.
• Selleys 655FC Fast Cure Adhesive Sealant — fast curing and versatile, for bonding and sealing construction or automotive surfaces.
• Selleys 670 Fast Grab Adhesive Sealant — advanced hybrid technology, a high strength elastic adhesive sealant with fast strength build-up, designed for applications where immediate grab is required.
• Selleys 675 Crystal Fix Adhesive Sealant — advanced hybrid technology, a multipurpose crystal clear elastic adhesive sealant for where an invisible bond is necessary.

"Immediate grab" is the distinction that decides whether 670 is the right product. Grab is the initial holding power before the sealant has cured — the difference between a component that stays where you put it and one that has to be propped or taped while it sets. Where you are bonding something vertical and cannot brace it, grab is the specification that matters, not final strength.

The 600 series has its own overview at Selleys 600 Series, which covers the professional sealants and hybrid sealant adhesives in the family.

If the bead must not be visible

Two products cover this, and they are not the same thing. Selleys All Clear gives a crystal-clear finish and is described for windows, showers and other glass surfaces, providing a water-tight seal. Selleys 675 Crystal Fix is a crystal clear adhesive sealant, designed for where an invisible bond is necessary.

The choice between them is whether you need the joint bonded. If you are sealing, All Clear. If you are sticking something down and want the join invisible, 675.

If the joint has to be painted

Selleys Paint Over Sealant is described as a paintable sealant that will not crack or shrink, for filling gaps and cracks in walls and ceilings.

This is not an optional preference. Many silicones cannot be painted at all — paint beads up and will not form a film on the cured surface — so if the joint is going to disappear under a coat, choosing a sealant designed for it at the outset saves cutting the bead out later.

If you want a general-purpose silicone with mould protection

Selleys Pro Trade All Purpose Silicone is a high quality, 100% neutral cure silicone suitable for a range of applications, containing anti-fungicide to prevent mould and mildew growth in hot and humid conditions.

Neutral cure is the reason this is the safer default among general silicones: it does not release acid as it cures, which removes a compatibility question about the surrounding materials.

If you need a low modulus elastic joint sealant

Selleys Flexiseal Clear is a premium low modulus, multipurpose elastic joint sealant powered by advanced hybrid technology, suitable for a variety of sealing applications. Low modulus means it develops less stress as the joint moves, so it puts less load back into the surfaces it is bonded to.

Selleys Flexiseal FC Multipurpose Sealant & Adhesive is the sealant-and-adhesive version, described for sealing gaps and cracks around windows and doors, and in walls and ceilings.

If the application is a gasket or electrical

Selleys Silicone 401 RTV was developed for form-in-place gaskets and adhesive sealing, and for electrical insulating applications in manufacturing and plant work. It is a specialist product, not a general household sealant.

The safety divide in this category — four bands, not one

"Multi-purpose sealant" is a job description, not a chemistry. The products above are silicones, silyl-modified polymers, a polyurethane and a solvent-borne copolymer, and their hazard classifications spread across four distinct bands. Two of them would need to be declared if you carried a boxful in a van; one carries nothing at all.

Band	Products	Classification
Danger — and Dangerous Goods	All Clear	H225 (highly flammable liquid and vapour), H315, H319, plus narcotic-effects Category 3. Dangerous Goods Class 3, UN 1133, Packing Group II — by road, rail, sea and air
Danger — eye damage	Silicone 401 RTV	H318 (causes serious eye damage — Category 1), H315
Warning	650 FC, 655 FC, 670 Fast Grab, Paint Over Sealant, Pro Trade All Purpose Silicone	H317 and/or H319 — skin sensitisation and eye irritation. 670 carries H319 only; Paint Over carries H317 only; Pro Trade adds H227 (combustible). None are Dangerous Goods
Not classified	675 Crystal Fix	Not classified as hazardous. No hazard category, no statement, no signal word, not Dangerous Goods
Classified, detail not published here	Flexiseal FC	Selleys states it is classified as hazardous and contains isocyanates, and requires gloves and safety glasses. No GHS breakdown is published — see product page

All Clear is the one that will surprise people, and the reason is transport rather than health. A cartridge of sealant does not look like a Class 3 flammable liquid, but UN 1133 Packing Group II is the same transport entry as a solvent-borne contact adhesive. Nothing else in this category is declared for transport at all. If you keep sealant in a hot vehicle, in a storeroom with an ignition source, or you are the person who has to answer a dangerous-goods question about what is in the van, All Clear is the one to know about.

Silicone 401 RTV is the only product here with a Category 1 eye classification. The difference between H319 and H318 is not a matter of degree in the way the wording suggests: irritation is expected to settle, Category 1 damage may not. An acetic-cure silicone squeezed from a cartridge at chest height, in a confined gasket or electrical enclosure, is exactly the geometry that puts material in an eye. Goggles, not glasses, for that one.

675 Crystal Fix is the answer to the question this category could not previously answer. If you want an adhesive sealant for a nursery, a confined space, a ward, a food area, or simply a job where you would rather not deal with a classification at all, it is the only product here that carries none. That is not marketing: it is the absence of solvents and the absence of isocyanates, and it is the direct counterpart to Flexiseal FC, which is a polyurethane and is classified hazardous precisely because it contains them. Not classified does not mean inert — wash your hands, keep it out of your eyes, and treat sanding a cured bead as a dust job — but there is nothing on the label to work around.

Among the Warning-band products, H317 is the classification that should change behaviour. Skin sensitisation is not irritation. It is the immune system learning to react to the material: generally permanent once acquired, set off afterwards by much smaller exposures, and built up through repeated casual contact rather than one bad incident. Forcing sealant into a joint with a finger, job after job, is how it is acquired. 670 Fast Grab is the one Warning-band product with no sensitisation classification at all — H319 only — which is worth knowing if you already react to sealants.

Comparing the sealants on the numbers that decide the job

Two figures decide most of these choices: how long the bead stays workable, and how much joint movement it will tolerate before it tears.

Product	Skin time	Movement capability	Service temperature
Silicone 401 RTV	5 minutes	See product page (100% elastic recovery)	−60°C to +205°C (260°C intermittent)
Pro Trade All Purpose Silicone	4–8 minutes	±25%	−60°C to 170°C
670 Fast Grab	~15 minutes	±20%	−40°C to +100°C (short term 180°C)
675 Crystal Fix	~15 minutes	±20%	−40°C to +90°C (short term 120°C)
All Clear	15 minutes (weather dependent)	See product page (500% elongation)	−10°C to +80°C
650 FC	20 minutes	±20%	−40°C to +70°C (brief points at 100°C)
655 FC	20 minutes	See product page	−40°C to +70°C
Flexiseal Clear	40 minutes	±12.5%	−20°C to 70°C
Flexiseal FC	Tack free 55–65 minutes	±25%	−40°C to +90°C (brief points at 120°C)
Paint Over Sealant	No technical data sheet is published — no cure rate, movement or service temperature figures exist for this product. See product page

The two products sharing the Flexiseal name sit at opposite ends of the movement range. Flexiseal FC is rated at ±25%, the highest figure in this category; Flexiseal Clear is rated at ±12.5%, the lowest, and half of most of its neighbours. A buyer choosing on the name alone, for an external joint that has to move, can land on exactly the wrong one. If the joint moves, that ±12.5% is the number to check, and the clear product is not the answer.

Skin time runs 4 minutes to 40 across this shelf, and it is the figure that decides how you work rather than how good the result is. The silicones skin in under ten minutes, which means tooling as you go, in short sections. The SMP adhesive sealants give you 15–20 minutes, and Flexiseal Clear 40, which is what makes a long unbroken run practical. A crystal-clear bead is also the one most punished by re-tooling a partly skinned surface, so the 40 minutes is not a luxury on that product — it is what the finish depends on.

Paint Over Sealant has no published technical data at all, and that is a specification in itself. There is no cure rate, no hardness, no elongation, no service temperature and no movement figure for it anywhere. It is bought for one property — paintable in an hour — and for an interior joint that does not move, that is a fair trade. For an external expansion joint, a joint with a temperature range to meet, or anything a specification has to be written against, choose a product that publishes the number instead of assuming this one would pass.

On heat, note how far apart the ends are: Silicone 401 RTV holds to 205°C continuous while 650 FC and 655 FC stop at 70°C. Both are sold as multi-purpose. A flue collar, a heater surround or a hot-water line is a 401 job, and the structural adhesive sealants are not alternatives for it at any bead size.

Common questions

What is an adhesive sealant?

A product that bonds the two surfaces together as well as sealing the joint between them. The 600 series products in this range are described as adhesive sealants; a plain sealant only closes the gap.

What is the difference between 650FC, 655FC, 670 and 675?

650FC and 655FC are fast cure adhesive sealants, with 655FC described for construction or automotive surfaces. 670 is built for immediate grab. 675 is crystal clear, for where an invisible bond is necessary.

Which one do I use if I cannot brace the parts while they set?

670 Fast Grab, which is designed for applications where immediate grab is required and has fast strength build-up.

Which sealant can I paint over?

Paint Over Sealant, described as paintable and as not cracking or shrinking. Do not assume the others take paint.

Which is the clear one for a shower screen?

All Clear is described for windows, showers and other glass surfaces with a water-tight seal. For a permanently damp shower recess, also compare the mould-resistant products in wet area silicone.

Does any product here resist mould?

Pro Trade All Purpose Silicone contains anti-fungicide to prevent mould and mildew growth in hot and humid conditions.

What does low modulus mean?

The sealant develops less stress as it stretches, so it transfers less load into the substrate at the bond line. Flexiseal Clear is the low modulus product in this section.

Is Silicone 401 RTV a general-purpose sealant?

No. It was developed for form-in-place gaskets, adhesive sealing and electrical insulating applications in manufacturing and plant environments.

Where do I find sealants for a specific location?

Use the location-based sections: wet area, roofing, construction, glass and fire rated.