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Boat Painting Weather

Article from Interlux website

Whether or not to paint depends on the humidity and dew point. The rule of thumb is not to paint when the Relative Humidity is above 85% or when the Dew Point is closer than 6 degrees to the ambient air temperature. But how many of us really understand exactly what this all means, and if we do, how do we actually measure them?

This article will explain what humidity and dew point are; and how they can affect your paint finish.

What is water vapor?
The first question we must ask ourselves is; what is water vapor? For the purpose of this article we should consider that water exists in various forms like ice, rain, mist, steam and clouds. It can also exist in vapor form where it cannot be seen. Relative humidity relates only to the vapor phase and not to any other form of water/air combination.

What is Relative Humidity?
Relative humidity (RH for short) is a ratio of how much water vapor is in the air compared to the maximum the air can hold at any specific temperature. Imagine that the air contains only half the maximum level of water vapor it could hold, and then the RH would be 50%. If it contained only a third of the maximum amount then the RH would be 33%. If the space contained the absolute maximum amount of water vapor then it would have a RH of 100%.
RH is a measure of water vapor and does not take into account any form of water present such as rain. On a rainy day RH measures the level of water vapor in the air between the raindrops so when it is raining the RH is not 100% as some people may think.

What is dew point?
We have all experienced dew point at one stage or another. Condensation on windows and wetness on the outside of a glass of cold beer are two obvious examples. Dew point is the temperature at which the air can no longer hold it's water vapor and a percentage of it starts to form water droplets i.e. condensation. This occurs because the lowering of the air temperature reduces the amount of energy available to keep the water in vapor form.
For example, a window forms condensation when the outside night air cools the glass down and the humidity in the house is high enough that some will condense out on the inside glass surface. To be able to paint a surface and avoid condensation forming, it is a rule of thumb that the surface temperature of the object you are painting must be around 6 degrees above the dew point.

What can cool a surface down?
To cool a surface down requires heat (energy) to be removed from that surface. When you apply a coat of paint to a surface the paint will absorb heat (energy), which then gives the solvents sufficient energy to evaporate from the paint film. This process continues until all the solvent has evaporated. The removal of energy from the object cools the surface down. You will experience this if you spill gasoline, solvent or alcohol on your skin, as it evaporates it cools your skin down.

Remember that surface temperature is different then ambient air temperature and must be measured with a surface thermometer to decide whether the surface is high enough above the dew point to allow you to paint. For example a steel hull will be cooler on the shady side compared to the sunny side. In the morning the sunny side will generally be lower than the ambient temperature where as in the afternoon it may be higher.

What is relationship between relative humidity and dew point?
The following chart shows a range of ambient temperatures from 86 degrees down to 41 degrees Fahrenheit. Against these readings are various relative humidity and calculated dew points for that ambient temperature. You can see that as the relative humidity drops so does the dew point. This makes sense because as the RH drops, the temperature must drop further to make a surface form condensation.

temp °C
temp °C
temp °C
82.29 62
82.27 53
81.61 44
80.92 35

You will notice that whenever the relative humidity is 100% the ambient temperature and the dew point are the same. This again makes sense because at this level of humidity the air is holding the maximum amount of water vapor it can possibly hold and any slight decrease in temperature would lead to an immediate deposition of moisture onto the surface.

So when can you paint?

  • Surface temperature should be 6 degrees above the dew point.
  • Generally paint when temperatures are rising to avoid getting caught by dew point.
  • Do not apply paint above 85% RH.
  • Preferably apply paint, especially two part polyurethanes when relative humidity levels are at or below 70%. One reason for this is that isocyanates curing agents react with moisture.
  • If you have sprayed walls and floors with water to lay down any dust, check that you're RH has not increased.
  • Keep within the temperature guidelines for application as stated on product data sheets remembering that the best and easiest application temperatures are generally those in the middle of the recommended ranges i.e. temperate conditions rather than extremes.

If you do not adhere to the above rules the following could result; yellowing of white paints, loss of gloss and you may also experience the paint not fully drying, causing the finish to remain soft.

Good painting
Original article from Interlux website

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