Whether to paint depends on the Weather

Weather plays an important part of our lives and even more so when it comes to painting. When to paint and when not to is an important choice if perfect finishes are required. There are rules of thumb, which state not to paint when the Relative Humidity is above 85% or when the Dew Point is closer than 3 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 try to explain in simplistic terminology what humidity and dew point is and how they inter-relate.

What is water vapour?

The first question we must ask ourselves is what is water vapour. For the purpose of this exercise we should consider that water exists in various forms. As a solid mass it exists as liquid water, ice, and droplets such as rain, mist, steam and clouds. It can then exist in vapour form where it cannot be visibly seen.

Relative Humidity relates only to the vapour phase and not to any other form of water/air combination.

What is Relative Humidity?

Relative Humidity (RH for brevity) is a ratio of how much water vapour is in the air compared to the maximum it could hold at any specific temperature.

Imagine that the air contains only half the maximum level of water vapour it could hold, 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 vapour then it would have a RH of 100%.

RH is a measure of water vapour and does not take into account any discreet form of water present such as rain. On a rainy day RH measures the level of water vapour in the air between the raindrops so when it is raining the RH is not 100% as some people may think.

How do we measure RH?

There are electronic instruments available and these vary in accuracy and reliability.

A standard piece of equipment called a Whirling Pschycrometer is one of the most reliable instruments, not requiring any batteries, and only taking around 15-20 seconds to obtain a reading. It is like a football rattle and is simply a holder with two thermometers, one of which has its bulb end exposed to the air and the other end covered with gauze which is kept wet via a small inbuilt container topped up with water prior to use. The unit is spun around for 15 - 20 seconds and the two thermometer readings are taken.

Both thermometers will read differently with the wet one reading lower by just a few degrees. The wet thermometer gives a lower reading because when it is spun around some of the water on the gauze evaporates which cools the thermometer down. Just how much water evaporates depends on the RH at the time. If the RH is high then less water will evaporate and if it is low, then more will evaporate.

A chart supplied with the instrument allows you to find the Relative Humidity using the temperature reading of the dry thermometer and the difference in reading between the two thermometers.

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 ones.

Dew Point is the temperature (in the paint industry we are normally concerned here with surface temperature) at which the air can no longer hold it’s water vapour and a percentage of it starts to form discreet 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 vapour form.

If for example you take a surface such as the hull or deck of a boat and start to cool it down, at some point condensation will start to form on the surface because the air close to that surface has also been cooled down. This is exactly the same as occurs at home when 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 3 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. This is how a refrigerator works for example.

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 feel the same effect if you spill petrol, solvent or alcohol on your skin, as it evaporates it cools your skin down.

Remember as well that surface temperatures are different to ambient air temperatures and must be measured with a surface thermometer to enable decisions to be made as to whether the surface is high enough above the Dew Point to enable painting to start. Obviously, 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 whereas 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 30 down to 5. Against these readings are various relative humidity’s and calculated dew points for that ambient temperature. You can see that as the Relative Humidity falls so does the Dew Point. This makes sense because as the RH falls, the temperature must drop further to make a surface form condensation.

temp °C
temp °C
temp °C
82.29 17
82.27 12
81.61 7
80.92 2

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 vapour it can possibly hold and any slight decrease in temperature would lead to an immediate deposition of moisture onto the surface.

If you now look at the yellow highlighted squares you will see that they all have a Dew Point around 3 degrees below the ambient temperature and they all have a Relative Humidity around the 81 to 84% level.

Hopefully now the penny (or should that be a cent?) has fallen and it starts to become clear why we should as a general rule not paint when the RH is greater than 85%. If the RH is greater than 85% then your surface temperature (to be 3 degrees higher than the Dew point) has to be equal or higher than the ambient air temperature. Possible, but generally speaking unlikely if painting outside.

Lets now go through a typical painting scenario. Imagine the air temperature is 20°C and we measure the RH as being 49%. The temperature of our steel hulled boat we want to paint is 11°C on one side and 17°C on the other. Question is can we paint it?

Looking at the above chart we see that at 20°C and 49% RH the Dew Point is reached at 9°C. Our steel needs to be at least 3 degrees higher than dew point so it needs to be around 12°C minimum. Answer - we should not paint the shaded side but we can the sunny side.

So when can we paint?

  • First rule as said before is that the surface temperature of what we want to paint should be 3 degrees above the Dew Point.

  • Generally paint when temperatures are rising to avoid getting caught by Dew Point. In some circumstances, if trying to coax paint or resin into a timber surface for example, applying product when that surface is in the stage of cooling down may help improve the products ability to penetrate. But be aware of Dew Point conditions.

  • Do not apply paint above 85% RH.

  • Preferably apply paint, especially two pack polyurethanes when RH levels are at or below 70%. One reason for this is that isocyanate curing agents react with moisture to various degrees. If this happens with your paint to any large degree then an improper cure can result. In the worst-case scenario yellowing of white paints and premature loss of gloss may occur.

  • If you have sprayed walls and floors with water to lay down any dust, check that your local RH has not increased over the general RH outside.

  • 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.