You applied a biocidal cleaner to your driveway in mid-October. It's now late October and the surface looks exactly the same. Nothing appears to have happened.

Before you call the supplier, or conclude the product doesn't work, read this. The chemistry is almost certainly working, it's just working significantly more slowly than it would in August. Understanding why helps you manage expectations, interpret what you're seeing, and make smarter decisions about when to treat and when to wait.

The Science Behind the Slowdown: Arrhenius Rate Law

Chemical reactions slow down at lower temperatures. This isn't specific to cleaning products, it's a fundamental principle of chemistry described by the Arrhenius rate law, which states that as temperature decreases, the rate of chemical reactions decreases exponentially.

For most biocidal reactions in the temperature range relevant to UK autumn and winter conditions, the practical rule of thumb is:

Every 10°C drop in temperature approximately halves the reaction rate.

This means:

• A treatment applied at 20°C that produces visible results in one week
• Applied at 10°C, will take approximately two weeks to produce the same result
• Applied at 5°C (borderline conditions), will take four weeks or more

This isn't a sign of product failure. It's temperature-dependent chemistry doing exactly what chemistry does.

How Cold Affects Biocidal Cleaners: Two Mechanisms

When you apply a biocidal cleaner like GK Pro, GK Path & Patio Cleaner, or GK Driveway Cleaner to a cold surface, two parallel mechanisms slow the result.

Mechanism 1: The Target Organisms Slow Down Too

Mould, algae, and lichen are biological organisms. Like all living things, their metabolic rate, or the rate at which their cells process chemicals, decreases in cold conditions. Cold-stressed biological cells absorb compounds less actively and their cellular processes slow significantly.

This has a paradoxical effect: a biocide works by penetrating the cell wall of the organism and disrupting its internal chemistry. When the organism's cells are metabolically slowed, the uptake of biocidal compound is reduced. The organism is harder to kill, not because it's stronger, but because it's already in a semi-dormant state.

The biocide will still penetrate and kill. It will just take longer.

Mechanism 2: The Biocidal Chemistry Itself Slows

The active ingredients in professional exterior biocides, DDAC (Didecyl Dimethyl Ammonium Chloride) in GK Pro, surfactant-based formulas in the surface-specific cleaners, work by disrupting the cell membranes of biological organisms. This disruption is itself a chemical reaction, and chemical reactions are temperature-dependent.

At 15°C, DDAC disrupts cell membranes in a fraction of the time it takes at 5°C. The compound is still active and still killing, it's just operating at a reduced rate.

This is why GK's exterior biocidal products carry a recommended minimum application temperature of 8°C. Below 8°C, the combined effect of slowed organism metabolism and slowed biocidal chemistry means that dwell times become so extended, and environmental conditions (frost, freezing rain) so unstable, that reliable results cannot be guaranteed.

What Cold Does to Sealers: A Different Problem

Biocidal cleaners slow down in cold conditions. Sealers have a more serious problem: they can fail permanently if applied outside their temperature window.

Stone Sealers (Minimum 10°C)

GK Invisible Stone Sealer requires a minimum application temperature of 10°C for both the surface and the ambient air. This is a higher threshold than the biocidal cleaners, and the reason is chemistry.

Stone sealer works by penetrating into the pores of the material and cross-linking to form a water-repelling barrier within the substrate. This cross-linking reaction, the chemistry that makes the sealer work, requires a minimum temperature to proceed. Below 10°C, the cross-linking reaction slows to a point where it may not complete before conditions change (rain, dew, frost), resulting in a sealer that hasn't properly cured.

The result isn't just poor performance; it's a surface that appears sealed but isn't, with chemical residue in the pores that may actually interfere with a future sealing attempt. Failed sealing is different from slow biological kill: you can retreat a surface that a biocide has treated slowly, but you may need to strip out incomplete sealer before a new application will work correctly.

In practical terms, 10°C as a minimum means late October and beyond becomes a genuine window concern in most of the UK. Surface temperature and air temperature matters; a shaded north-facing driveway may be below 10°C even when the air temperature reads 12°C.

Timber and Wood Sealers (Minimum 5°C)

GK Invisible Wood Sealer carries a lower minimum temperature of 5°C, making it usable in cooler conditions than stone sealer. The formulation chemistry is different, with a lower-temperature cross-linking mechanism that allows application deeper into the autumn.

But 5°C is still a hard lower limit. Timber sealers applied below this temperature may not form a complete film over the wood grain, leaving gaps that moisture and biological growth can exploit.

The UV Factor: Why No-Rinse Results Look Slower in Low Light

Several GK products, including GK Pro used on roofs and large surface areas, work on a no-rinse, soft-wash basis. The product kills the biological growth, and then rain and UV light break down and remove the dead material over time, clearing the surface progressively in the weeks and months after treatment.

This UV-assisted clearance mechanism is significantly slower in autumn and winter, for two reasons:

Reduced UV intensity: UV radiation is a primary driver of the breakdown of dead organic material on exterior surfaces. UK UV intensity in October–December is a fraction of summer levels, approximately 20–30% of June peak intensity. Dead algae and mould that might wash clear from a summer-treated roof in two weeks may persist for six to eight weeks on a surface treated in November.

Shorter photoperiod: Fewer daylight hours means less total UV exposure per day. Even on clear October days, the sun angle is low enough that UV intensity is substantially reduced compared to summer months.

If you've used a no-rinse product on a roof, large wall, or other surface in autumn and the dead material hasn't cleared yet, this is the most likely explanation. The organism is dead; the clearing is just taking longer than it would in summer. Manual low-pressure rinsing can accelerate this process if you're waiting on a result.

What Cold Does to Products in Storage

There's a third temperature effect that homeowners and trade users often overlook: what cold does to cleaning products sitting in a shed or van.

Water-Based Sealers and Emulsion Products

Water-based sealers, including GK Invisible Stone Sealer and GK Invisible Wood Sealer, are emulsion products. They consist of active polymer particles suspended in water. If these products freeze, the ice crystals that form rupture the emulsion. When the product thaws, instead of a uniform milky liquid, you'll find a lumpy, separated, or grainy product where the polymer particles have coagulated.

A product that has frozen and thawed cannot be reconstituted: The emulsion cannot be re-formed by shaking or stirring. The product must be discarded.

This means as overnight temperatures begin to fall, any water-based sealer stored in a garage, shed, or vehicle overnight needs to come indoors. The product doesn't need to warm up, it just needs to stay above freezing.

Biocidal Cleaners

Biocidal cleaners are less vulnerable to freeze damage than emulsion sealers, but they should still be stored above freezing temperatures wherever possible. Most professional biocidal formulas (including the DDAC-based GK Pro) will withstand freezing better than emulsion products, but freeze-thaw cycling can affect concentration, dilution characteristics, and in products containing surfactants, the surfactant structure.

As a general rule: Store all GK products above 5°C during winter months, and keep them in a frost-free location overnight once temperatures regularly fall below 5°C.

Month-by-Month: What's Still Realistic in the UK

This guide assumes typical UK average temperatures. Microclimates vary; north-facing surfaces, shaded areas, and properties in Scotland or upland regions will reach limiting temperatures earlier.

 

What to Do If You've Already Applied in Borderline Conditions

If you applied a biocidal cleaner in cooler-than-ideal conditions: Allow significantly more time than a summer application. Six to eight weeks is not unusual for a product applied in late October in cooler regions. The biology is being killed, it's just a slow process. Avoid rinsing or scrubbing prematurely; let the chemistry complete its work. In spring, re-assess whether a maintenance application is needed to finish what autumn began.

If you applied a sealer in conditions below the minimum temperature: Assess the result carefully. Run water across the surface; if it beads and runs off, the sealer has cured adequately. If water soaks in without beading, the sealer may not have formed correctly. In this case, wait for warmer conditions and consider a re-application. If the surface was already sealed previously, a partial failure of the new coat may still leave some protection from the previous treatment.

If a product has frozen in storage: Check the appearance and consistency. A sealer that has frozen will usually show obvious separation, graining, or a lumpy texture. Discard it, it cannot be restored. Order a replacement before spring application season begins.

Temperature Reference: All GK Products at a Glance

 

Frequently Asked Questions

My treatment was applied in October and it's now been six weeks - has it failed?

Not necessarily. A biocidal cleaner applied in cooler October conditions, particularly in the north of England, Scotland, or on a shaded north-facing surface, can take six to eight weeks to produce visible results, compared to one to two weeks in summer. The Arrhenius rate law means reaction rates roughly halve for every 10°C temperature drop. If conditions have been consistently cold since application, the product is still working, it's just working slowly. Assess in spring before concluding the treatment hasn't worked.

At what temperature should I stop using GK exterior cleaners?

The minimum recommended application temperature for GK's biocidal exterior cleaners (Path & Patio, Driveway, Render, Decking, Pro) is 8°C. Below this, the combined effect of reduced organism metabolism and slower biocidal chemistry means reliable results cannot be guaranteed. Check both air temperature and surface temperature; shaded surfaces on cold days may be several degrees below the air temperature.

Can I seal my patio in October?

It depends on your location and the forecast. GK Invisible Stone Sealer requires a minimum application temperature of 10°C for both the surface and the air. In early October in southern England, this is typically achievable on warmer days. In late October, particularly in northern England, Scotland, or on shaded surfaces, 10°C becomes increasingly difficult to guarantee. Check the surface temperature with a contact thermometer and ensure no frost is forecast for at least 24-48 hours after application.

What happens to the sealer if it gets cold before it's fully cured?

If temperatures drop below 10°C (stone) or 5°C (wood) before the sealer has fully cured, which requires 24 hours of suitable conditions, the cross-linking reaction may not complete. The result can be a sealer that has partially formed but doesn't provide full water-repellent protection. Monitor the weather forecast on application day and in the 24 hours following.

My stone sealer froze in the shed. Can I still use it?

No. Water-based sealers, including GK Invisible Stone Sealer, are emulsion products. Freezing ruptures the emulsion. When the product thaws, the polymer particles have coagulated and the product will not perform as intended. A frozen-and-thawed sealer should be discarded and replaced. To prevent this, store all water-based sealers in a frost-free location once overnight temperatures regularly fall below 5°C.

Does cold weather affect GK Pro's two-year residual action?

No. GK Pro's two-year residual biocidal activity refers to its action on already-treated surfaces; the DDAC that has bonded to the surface material continues to kill newly arriving spores throughout that period. Cold weather slows the rate of kill but doesn't deactivate the DDAC or reduce the residual period. A surface treated in autumn is still protected through winter; the residual action continues once temperatures recover in spring.

Is it worth treating in November at all?

For biocidal cleaners, late November is generally borderline to too cold across most of the UK, with temperatures frequently falling below 8°C even during the day in cooler regions. For severe contamination, an autumn treatment at marginal temperatures is better than no treatment; you're still killing biology, just slowly, and the residual action protects through winter. For routine maintenance, waiting until March or April is likely to give faster, more reliable results. For sealing, November is generally too late, wait until spring.

Summary

Questions about whether it's the right time to treat your specific surface? Get in touch with the GK team at sales@gkproductsuk.com or call 07557 111 297.