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
- 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.
Key points to note when applying
epoxies (e.g. Watertite, InterProtect®,
- 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
- 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
- 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