Winter storage: Sunny days, icy nights, why the dew point is important when applying antifouling and paint

The sun gets stronger during the day and the anticipation of the first sailing weekend increases. It's time to apply antifouling or renew the paintwork - provided the temperature is favourable.

Dew point: when moisture becomes an enemy

When applying antifouling, the temperature of the hull plays just as important a role as the ambient temperature. For the application of one-component products, manufacturers generally specify a substrate temperature of five degrees, for two-component paints or primers it is often 12 degrees or more. The dew point comes into play as soon as the body and the environment are not at the same temperature.

The dew point describes the temperature at which the water vapour contained in the air condenses. In practice, this means that the hull should always be at least the same temperature as the ambient air - ideally it should be three or more degrees warmer. If the hull is colder than the air, the moisture in the air will condense on it. If the antifouling is painted onto this film of water, it will adhere poorly and may peel off later. Condensation is particularly annoying when painting. Even if it sets in after application, it leads to a poor gloss or the paint becomes milky.

Don't start too early

As the dew point is directly dependent on the temperature, it has a diurnal variation. However, it should be noted that the air generally warms up much faster than a hull or keel that has been cooled through at night. The temperature difference between the air and the boat increases as the morning progresses and the risk of condensation increases. For this reason, you should not start painting in the morning or mid-morning, but wait until the temperature has exceeded its daily maximum. If the hull is still dry then, there will probably be no more moisture problems.

Practical tips for temperature control:

• Use a laser thermometer for the fuselage.
• Also measure the keel - it heats up more slowly than the rest of the boat.
• Avoid working in the morning; the torso cools down considerably at night.

Off into the warmth

Temperature not only plays a key role in the dew point. It also influences the viscosity of the paint and determines the evaporation of solvents and the curing process of paints and resins. For this reason, the material should not be left under the boat or in the car overnight. Temperatures drop rapidly after sunset, and in spring it is not uncommon for temperatures to drop below zero in the morning. A container that has been cooled down in this way needs hours to return to a workable temperature. You should therefore take the materials home with you and store them at 15 to 20 degrees.

More on the topic

• Renew antifouling, repair damage to the hull
• You can do these winter jobs despite the icy cold