Always check the weather

It sounds obvious, but when painting outside always check what the weather forecast will be during the preparation, application and drying phases of your project. If fair weather is forecast, you should also take into account the air and surface temperatures, humidity and dew point to help you decide whether to start or not.

Guidance notes

• Dew point is important when applying paint to a surface, as the evaporation of the solvent from the paint draws heat and/or energy from that surface, cooling it down.
• If conditions are right, condensation may form on the surface of the paint resulting in various problems.
• Relative humidity is important as air can only hold so much water or solvent vapor at any one time. So, as the relative humidity increases, the level of solvent vapor the air can hold reduces, meaning paint will effectively dry more slowly.
• Air and substrate temperature will affect the drying properties of any paint. If you don’t follow the recommended drying times it could result in coating failure, including improper drying, wrinkling and loss of adhesion.
• Always avoid extreme air or temperature conditions. Most products are tested across a range of temperatures to discover the drying times and application characteristics. Further information about weather considerations can be found on the product datasheets.
• Low temperatures will increase drying times. Always check the ‘through-dry’ of each interim coat, before sanding or overcoating.
• Sanding too early can cause the paint to wrinkle under the sand paper. In some cases even tearing or gouging into the paint film making refurbishment difficult.
• Sanding before the paint film is ‘through-dry’ can also clog the sandpaper, meaning more sheets are needed to complete the task.
• Overcoating too early can cause wrinkling, blistering and loss of gloss in the finished paint job.
• High temperatures will reduce drying times, but they can make application more difficult, as product flow and leveling can be compromised – particularly when applying finishes or varnishes. Where appropriate, you’ll find thinning recommendations to help on labels and datasheets.
• Don’t paint in direct sunlight, or when the substrate itself is excessively warm. The residual heat of the substrate can adversely affect the application and drying properties of any paint product. This can result in poor flow and leveling, rapid drying, cracking and loss of gloss. We’d recommend measuring the surface temperature with a surface thermometer.
   

What is relative humidity?
 

Relative humidity, or ‘RH’, measures the amount of water in the air in vapor form, comparing it to the maximum amount of water that can be held at a given temperature.
For example, if the RH is quoted as being 50% at 23°C, this would imply that the air contains 50% of the maximum level of water vapor it could hold at 23°C. 100% RH indicates that the air is at maximum saturation.
Relative humidity can be measured using a hygrometer. You could also use a hygro thermometer that measures both temperature and humidity.
Tip: Never apply paint above a maximum relative humidity of around 85%. At that level you’ll reach the dew point regardless of surrounding temperatures.
 

What is dew point?
 

When humid air comes into contact with cooler air, or a cooler surface, the water vapor will turn into water droplets, i.e. condensation. When this occurs on a surface, it is referred to as the ‘dew point’.
When painting, it’s essential that you’re able to paint a surface without condensation forming during the application and drying stages. The optimum surface temperature for paint application is normally around -14°C above the dew point. However, some products may be more or less tolerant. If in doubt, it’s always worth checking the relevant technical datasheet.
The dew point can be calculated using the relative humidity and the air temperature. But to make life easier, you can find many dew point lookup tables on the web.
 

When applying finishes and varnishes
 

• Dry, well-ventilated conditions are preferable when applying finishes or varnishes. While gentle air movement will assist the drying process, a dust-free environment is critical to achieving a good quality gloss finish. Always avoid painting in windy conditions.
• The effects of dust contamination can be reduced by sanding lightly between each coat. Once done, remove residual dust by wiping down the surface with a suitable solvent or tack rag, and allow to dry before applying the next coat. This will also help improve the initial aesthetics.
• Avoid applying two-part finishes or varnishes late in the afternoon or when relative humidity exceeds 80%, as these products are particularly sensitive to moisture. Condensation during application can affect the chemical cure of these products resulting in loss of gloss.
• When painting or varnishing wood, avoid applying it if the ambient temperature is increasing (or predicted to increase) significantly. Rising temperatures cause wood to expand, which can lead to blisters forming in the paint or varnish film. Tip: Apply when temperature is falling, as the wood will absorb the paint or varnish better giving overall better results.