Applying Two-Pack Polyurethane Paint in Cold Weather

Two-pack polyurethane topcoats are a popular choice for marine painting for their hard-wearing, high-gloss finish. Applied in the right conditions, they provide long-lasting protection and an appearance that holds up season after season.

During the autumn and winter months, however, application becomes considerably less forgiving. Lower temperatures slow the curing process, while high humidity and condensation increase the risk of defects such as blooming. Once blooming has formed, sanding and recoating may be needed to restore the finish, following the manufacturer’s repair guidance.

Blooming is one of the most common issues customers raise with us at this time of year, and in almost every case it can be prevented. This guide explains why cold and damp conditions affect 2-pack polyurethane, how to assess whether conditions are suitable for painting, and what to do if blooming has already occurred.

How winter conditions affect two-part polyurethane paint

Unlike single-pack paints, two-pack polyurethane requires a separate hardener to be mixed with the base before application. The mixture then cures through a chemical reaction between the base and hardener. That reaction is sensitive to two environmental variables that are far more prevalent in the winter:

• Moisture: The hardener in a two-pack polyurethane contains isocyanates, which react readily with water. Moisture on the wet film can cause a dull surface, sometimes with fine bubbles or pinholes, but blooming does not necessarily mean that the coating is softer or has lost its protective properties once fully cured.
•  Low temperatures: Cold slows curing and solvent evaporation, extending the time during which the fresh coating needs protection from moisture. It can also thicken the paint and make it harder to apply evenly.

Blooming is one possible consequence of unsuitable conditions. Cold can also delay curing or make the coating difficult to apply evenly, even where no cloudy appearance develops.

Checking the dew point and application conditions before painting

One of the most important factors to understand when applying a 2-pack polyurethane topcoat in winter is the dew point: the temperature at which moisture in the air begins to condense onto a surface. The higher the relative humidity, the closer the dew point sits to the air temperature, leaving far less room for a hull or deck to cool before condensation starts to form.

If the substrate reaches the dew point during application, or at any point while the coating is still vulnerable to moisture, microscopic condensation can form on the fresh paint. This moisture is often too fine to see, but it interferes with the reaction between the base and hardener and can result in reduced gloss, blooming, blushing or other surface defects, even where the surface looked perfectly dry when the paint was first applied.

The checks below will help you establish whether conditions are suitable for your chosen 2-pack polyurethane topcoat before you start painting, and whether they are likely to remain suitable while it cures.

Follow the temperature limits for your chosen paint

Because application limits vary from one product to the next, the technical data sheet and application guide for your exact topcoat should always be your starting point, rather than general advice for polyurethane paints. 

Check the minimum air and surface temperatures alongside any requirement for the temperature of the paint itself when it is mixed and applied, as these are separate measurements, and a coating that can cure on a cold surface may still need warmer paint to mix and apply properly.

• Jotun's Hardtop XP application guide, for example, specifies minimum air and surface temperatures of 5°C, but recommends that the base and hardener are at 18°C or above when mixed.
• The SML Lustre polyurethane topcoat data sheet, by comparison, should not be applied below 10°C.

This difference shows why one product's limits should never be assumed to apply to another, and why general advice for automotive lacquers, decorative paints or wood varnishes should not replace the instructions for your chosen coating.

Measure surface temperature and relative humidity

A weather forecast is useful for planning, but it cannot tell you the temperature of the hull or structure you intend to paint, which may be noticeably colder than the surrounding air, particularly in shaded areas or unheated buildings. Therefore, before starting you should measure the air temperature and relative humidity near the work, together with the substrate temperature, then use the air readings to calculate the dew point and compare it with the substrate temperature.

• Surface thermometer: measures the temperature of the substrate.
• Thermo-hygrometer: measures air temperature and relative humidity close to the work.
• Coating inspection meter: combines these readings and calculates the dew point automatically. Without one, the air temperature and humidity readings can be entered into a dew point calculator.

Take readings from representative areas, including colder sections such as shaded sides and horizontal surfaces where dew settles more readily, and repeat them during the job, particularly if temperatures are falling or the weather is changing.

Check the margin above dew point

As a general guide, the substrate should be at least 3°C above the dew point, which is the margin Hardtop XP specifies. With a dew point of 6°C, for example, the surface would need to be at least 9°C, while still meeting every other application requirement on the data sheet. The exact requirement does vary between products, however, so follow the figure stated for your chosen topcoat rather than treating 3°C as a universal rule.

During winter, it is not enough to check the margin only when you start painting. Consider how temperature and humidity are expected to change through the afternoon and evening, as a surface that sits comfortably above the dew point in the early afternoon may be much closer to condensation conditions once the sun begins to drop, while the coating is still curing.

Reduce the risk of blooming during winter application

Confirming that conditions are suitable is only the first step, as the way the job is planned, prepared and protected determines whether they remain suitable for long enough for the coating to cure. The guidance below covers each stage, from choosing when to paint through to protecting the finish once it has been applied. 

Choose a painting window that allows for drying

Plan the work for a dry period with suitable temperatures, allowing enough time for the coating to dry sufficiently before evening conditions deteriorate. Avoid starting late in the day when the surface temperature is falling rapidly, particularly if dew, mist or rain is expected.

Use the manufacturer's drying information for the temperature of your job, rather than assuming that a time quoted for warm conditions will apply in winter. Jotun Hardtop XP, for example, reaches walk-on dry in 7 hours at 23°C but takes 14 hours at 10°C and 24 hours at 5°C, and Jotun advises that blushing may occur if the wet coating is exposed to moisture, or to relative humidity above 85%, before it reaches that stage. The coating does not necessarily need to reach full cure before nightfall, but it must remain within the specified conditions and protected from moisture for the period the data sheet requires.

Take particular care with decks, cabin roofs and other horizontal surfaces where dew can settle readily. If suitable conditions cannot be maintained, postpone the work or arrange an appropriately controlled environment, such as a heated and dehumidified building. 

Prepare the surface and mix the coating correctly

Follow the preparation instructions for the substrate and existing paint system, removing contamination and sanding dust before confirming that the surface is completely dry. 

Measure the base and hardener in the correct ratio, mix as instructed and observe any induction time specified on the data sheet (the waiting period required between mixing and application). Keep within the stated pot life, which is the usable time of the mixed paint and shortens as temperatures rise.

The paint itself may also need conditioning before use. The SML Lustre data sheet recommends keeping the product at 15–20°C before and during application, as below 15°C the base and hardener thicken considerably, while Jotun recommends that Hardtop XP's base and hardener are at 18°C or above when mixed. These recommendations concern the temperature of the paint itself and do not replace the checks on air and surface conditions.

Use only the recommended thinner and application method, keeping within the permitted thinning quantity rather than adding extra thinner or hardener to compensate for cold weather. 

Maintain ventilation and protection after application

Continue checking conditions after painting, remembering that shelter from rain does not guarantee protection from condensation, as moisture can still form inside an unheated building or beneath a temporary cover as temperatures change.

Touch dry, ready to overcoat and fully cured describe different stages, so a surface that feels dry may still need protection. Follow the product guidance when deciding whether it can be recoated or exposed to service conditions.

Provide appropriate ventilation for solvent vapours and follow the safety data sheet when selecting protective equipment or arranging heating. Avoid gas or paraffin heaters in an enclosed space, as burning fuel releases water vapour that raises the humidity around the fresh coating, and an open flame near solvent-based paint is a fire risk. Spraying 2-pack polyurethane requires additional controls, including air-fed respiratory protection, so guidance for brush or roller application should not be treated as spray safety advice.

Dealing with your finish affected by blooming

Before sanding or applying another coat, assess the coating and note the product used, when it was applied and the conditions during drying. Photographs can help us advise on the likely cause and the appropriate repair.

Blooming does not automatically mean that protective performance has been lost. Jotun's Hardtop XP guidance states that blushing affects gloss and can fade bright colours but does not affect the coating's protective properties once it is fully dried and cured, although this should not be assumed for every product or defect.

Where the coating is firm and the problem affects only its appearance, abrading the surface until the bloom is removed and then recoating in suitable conditions may be appropriate. Confirm that this approach is suitable for your paint, and that the coating has dried sufficiently, before beginning the repair.

If the coating remains soft or tacky, or shows bubbling or poor adhesion, seek advice before proceeding. Check the preparation requirements and recoating window and correct the application conditions before applying the replacement coat.

Advice for your winter painting project

If you are unsure whether conditions are suitable, contact SML before mixing your paint. Tell us which coating you are using, what surface you are painting and whether you will be working outdoors or under cover, along with any temperature and humidity readings.

Explore our two-pack topcoats, or call 01285 862132 for advice on preparation and application.