Always check the weather!

When painting outside, always check what weather conditions are anticipated during the preparation, application and drying phases of any project. Should fair weather prevail, whether or not to commence painting will then depend on the air and surface temperatures, humidity and dew point. You may find the following hints and tips helpful when planning your project – further, product-specific guidelines can be found on individual product labels and data sheets.

General 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. Failing to observe the recommended drying times can result in coating failure, including improper drying, wrinkling and loss of adhesion.
  • Always avoid extreme air or temperature conditions; Interlux products are tested across a range of temperatures, to ascertain the drying times and application characteristics of each product. Drying time recommendations are provided on our products labels; further information relating to weather considerations can be found on our product data sheets, available on our web site.
  • 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 sand paper, 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 can make application more difficult, as product flow and leveling can be compromised – particularly when applying finishes or varnishes. Where appropriate, thinning recommendations to help with higher temperature application are provided on labels and data sheet.
  • Do not paint in direct sunlight, or when the substrate itself is excessively warm, as 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. Surface temperature can be measured using 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 73°F, this would imply that the air contains 50% of the maximum level of water vapor it could hold at 73°F. 100% RH indicates that the air is at maximum saturation.

When humid air comes into contact with cooler air, or a cooler surface, the water vapor will turn into water droplets. When this occurs on a surface it is referred to as the ‘Dew Point’.

Never apply paint above a maximum relative humidity of around 85% as at that level you will reach the dew point regardless of surrounding temperatures.

Relative Humidity can be measured using a hygrometer; a hygro thermometer will measure both temperature and humidity.

“What is ‘Dew Point’?”

We have all experienced ‘Dew Point’ at some stage. Condensation on windows is an obvious example. Dew Point is the temperature at which the air can no longer hold its water vapor and starts to form water droplets i.e. condensation. Condensation forms when air temperature lowers, reducing the amount of energy available to keep the water in vapor format. For example, a window forms condensation when the cool night air hits the outside surface of the glass, lowering the surface temperature and the humidity in the house is high enough to cause condensation on the inside of the glass surface.

When painting, it is essential that you are able to paint a surface and avoid condensation forming during the application and drying stages. The optimum surface temperature for paint application is normally around 6°F above the Dew Point; however some products may be more or less tolerant. When working with Interlux products, if in doubt, it’s always worth checking the relevant technical data sheet

Dew Point can be calculated using the relative humidity and the air temperature; however, Dew Point ‘lookup’ tables are widely available via the internet.

Key points to note 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 may be further reduced by sanding lightly between each coat, removing residual dust by wiping down with a suitable solvent and allowing 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 or due to overnight ambient temperature changes can affect the chemical cure of these products resulting in loss of gloss.
  • When painting or varnishing wood avoid applying if the ambient temperature is increasing (or predicted to increase) significantly. This is because rising temperatures cause wood to expand, which can lead to blisters forming in the paint or varnish film. A good tip is to apply when the temperature is falling, as the wood will better absorb the paint or varnish, giving better overall results.
  • Blistering

Key points to note when applying epoxies (e.g. Watertite, InterProtect®, Epoxy Primekote)

  • When curing in high humidity conditions, particularly at lower temperatures, epoxies can develop an ‘amine blush’ on the surface. This slightly sticky substance must be removed and can normally be washed off with soap and water. If the blush is not removed it can lead to the delamination of subsequent coats. Failure to remove the blush will also make sanding more difficult.
  • Blooming
  • High humidity conditions can reduce the amount of solvent evaporation during the drying/curing stages; with epoxies this can lead to a ‘soft cure’. As epoxy-based materials are generally applied at a higher film thickness, solvent can remain trapped in the film for many days leading to slow or poor final cure.
  • Although epoxies generally cure well in most conditions, when the temperature falls to 45°F or below, curing can slow or even stop. Remember to check both day and overnight temperatures whether working outdoors or in a shed.
  • Epoxy products usually respond well to a little heat; on cold days introducing a safe form of heating into the application area is well worth considering.