Which Selleys auto sealant do you need, and how do you choose?

The Selleys Auto Fix range is five sealants for vehicles and boats. They divide cleanly by the job: two are gasket products, one is a windscreen sealant, one is a marine sealant, and one is the general auto sealant. Once you know which of those four situations you are in, the choice is essentially made.

The one distinction inside the range that catches people out is temperature. There are two instant gasket products, and the difference between them is that one is specified for high temperature. If the gasket you are replacing sits anywhere near an engine's hot side, that difference is not a nicety — an ordinary sealant in a high-temperature location fails, and it usually fails as a slow leak rather than an obvious one.

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 five products the live sitemap lists under this path. Three of them carry no published description in the source, so this page names them and what their names indicate, without inventing properties for them. Check the individual product pages for their specifications before use.

The range as a whole is described in terms of sealing a car with a waterproof formula that gives a secure, lasting bond.

How to choose

If you are making a gasket

Selleys lists two instant gasket products. Auto Fix Instant Gasket is the standard version, and Auto Fix Instant Gasket High Temperature Sealant is the version specified for high temperature service.

An instant gasket product forms the gasket in place rather than requiring a cut or moulded one, which is what makes it useful for an assembly where the original gasket is unavailable or the mating faces are not perfectly flat. Match the product to the service temperature of the assembly and check the product page for the stated temperature limit before you commit to it.

If you are sealing a windscreen

Auto Fix Windscreen Sealant is the product named for windscreen work. Glass-to-body sealing is its own problem — the joint has to stay watertight through vibration and thermal cycling while remaining compatible with automotive glass and paint — which is why it is a separate product rather than a general one.

If you are working on a boat

Auto Fix Marine Sealant is described as keeping a boat safe from water damage, with a waterproof formula. Marine work is the most demanding case in this range because the seal is in continuous or repeated contact with water rather than occasionally wetted.

For general vehicle sealing

Auto Fix Auto Sealant is the general product in the range.

If you need a hybrid adhesive sealant instead

Selleys 655FC Fast Cure Adhesive Sealant sits in the multi-purpose range but is described as suiting bonding and sealing on construction or automotive surfaces, with fast curing. It is the option to compare when the joint needs structural bonding as well as sealing.

The products in this range

• Selleys Auto Fix Auto Sealant — the general auto sealant in the range.
• Selleys Auto Fix Instant Gasket — forms a gasket in place.
• Selleys Auto Fix Instant Gasket High Temperature Sealant — the high-temperature instant gasket product.
• Selleys Auto Fix Windscreen Sealant — for windscreen sealing.
• Selleys Auto Fix Marine Sealant — waterproof formula to protect a boat from water damage.

The safety divide in this category — and it runs the opposite way to expectation

Five tubes, all the same size, all silicone, sitting together on the same shelf. Two of them carry no hazard classification at all and three carry a Warning. The split is not random and it is not about how hard the job is — it follows the cure chemistry exactly, and it lands in the direction most people would guess wrong.

Group	Products	Classification
Acetoxy cure — not classified	Auto Fix Instant Gasket, Auto Fix Instant Gasket High Temp Red	Not classified as hazardous. No signal word, no hazard statements, no poison schedule, not Dangerous Goods
Neutral (oxime) cure — Warning	Auto Fix Auto Sealant, Auto Fix Windscreen Sealant, Auto Fix Marine Sealant	Warning. H319 (causes serious eye irritation) and H317 (may cause an allergic skin reaction). Not Dangerous Goods

The products that are safe for your paintwork are the ones that carry the health classification, and the products that carry no classification are the ones that will attack the paint. That is the whole category in one sentence, and it is the opposite of the intuition that a harsher product must be harsher in every direction.

The two gasket makers are acetoxy silicones. Acetoxy chemistry releases acetic acid as it crosslinks — the vinegar smell when you open the tube — which is why the pages above keep them away from painted surfaces and why neither belongs near trim. That acid is a hazard to the finish rather than to you, and it is not what a GHS health classification measures. Both come out unclassified, with only cyclotetrasiloxane at under 1% named in the composition.

The three neutral-cure sealants release an oxime instead, which is precisely what makes them usable against automotive paint, glass and trim. Oximes are the component class usually behind skin-sensitisation classifications in neutral-cure silicones, and all three carry H317 as a result. The safety data sheets do not attribute the classification to a named ingredient, so read that as the chemical explanation rather than a sourced one.

H317 is the one to change your habits over, not H319. Eye irritation is unpleasant and passes. Sensitisation is the immune system learning to react: generally permanent once acquired, triggered afterwards by exposures far smaller than the one that caused it, and built up through repeated casual contact rather than a single incident. Tooling a bead with a wet finger, over and over across a rebuild, is exactly the exposure pattern that acquires it. Wear gloves for the neutral-cure three; you can be more relaxed about the two gasket makers.

Worth noting within the neutral-cure group: the Windscreen Sealant declares its aminosilane at 1–10%, where the other Auto Fix sealants hold it below 1%. The classification is identical, but the dose is not.

Comparing the auto sealants on the numbers that decide the job

These five share a remarkable amount of their data sheet — the same 1.02 g/ml density, the same 5-minute skin time, the same 30-minute set, the same 24-hour cure, the same −20°C to 50°C application range and the same 25% movement capability. What separates them is heat and hardness.

Product	Cure type	Service temperature	Shore A	Elongation
Instant Gasket High Temp Red	Acetoxy	−40°C to 260°C; intermittent to 343°C	18	300%
Instant Gasket (black/grey)	Acetoxy	−40°C to 205°C; intermittent to 260°C	18	370%
Auto Fix Auto Sealant	Neutral	−40°C to 180°C	25	370%
Windscreen Sealant	Neutral	−40°C to 180°C	25	370%
Marine Sealant	Neutral	No technical data sheet is published for this product — see product page

The 80°C gap between the gasket makers and the general sealants is the number that decides where each one can go. A neutral-cure Auto Fix sealant tops out at 180°C. That is comfortably enough for body seams, lamp housings, boot and door seals and a windscreen, and nowhere near enough for a rocker cover or a component sitting against an exhaust manifold. The acetoxy gasket makers exist for that second job, and the High Temp Red goes further again — 260°C continuous with intermittent excursions to 343°C, against 205°C and 260°C for the standard black.

So the paint restriction and the temperature rating are the same decision seen twice. If the job is hot enough to need a gasket maker, you are inside the engine bay on bare metal, where an acetoxy cure is not a problem. If the job is on or near a finished surface, you are also, almost by definition, somewhere that never reaches 180°C. The two constraints line up, which is why the category works — but it also means that a job which is both hot and next to paintwork has no product here that satisfies both. That job needs a mechanical gasket, not a sealant.

The harder sealant is the more flexible one, which is not a contradiction. The neutral-cure products are Shore A 25 against the gasket makers' 18, yet both reach 370% elongation and both are rated at 25% movement. Shore A describes resistance to indentation, not stretch. The firmer surface is what you want on an exposed exterior seam that gets washed, brushed and knocked; the softer one is what you want compressed between two metal faces that are about to be bolted together.

The 5-minute skin time is shared across every product here that publishes one, and it is short. Tool the bead as you lay it rather than running the whole seam first and coming back — on a long body seam you will be tooling cured sealant by the time you return to the start.

Common questions

What is the difference between the two instant gasket products?

One is specified as a high temperature sealant and the other is not. Choose on the service temperature of the assembly, and check the product page for the stated limit.

Can I use the standard instant gasket near an engine?

Only if the temperature at that location is within its stated limit. Where it is not, the high temperature version is the product Selleys provides for that purpose. Do not assume the standard product will cope.

Can I use an auto sealant on a boat?

Selleys makes a dedicated marine sealant in this range, described for protecting a boat from water damage with a waterproof formula. Use that rather than a general automotive product for marine work.

Are these sealants waterproof?

The range is described in terms of a waterproof formula that ensures a secure bond that lasts, and the marine product is specifically described as waterproof. Check the individual product page for the exact claim on the product you are buying.

Why do some products here have no description?

Three of the five carry no published description in the source used to build this page. Rather than invent properties for them, this page names them and points you to their own product pages.

Is there a Selleys product for a car body repair rather than a seal?

Body repair is a different job from sealing and belongs to the filler range rather than here. Start at specialist fillers and putty.

Where do I find the rest of the sealant range?

Under sealants, which covers wet area, roofing, construction, glass, fire-rated and multi-purpose products.