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Technical Notes: Humidity

High humidity  |  Low humidity

Moisture cured polyurethanes, ie the Duracoat MCR and MCL ranges, and the Uroxsys Marine Clearcoat Range must have moisture in the air to cure, ideally between 50 and 75% relative humidity (R.H.).

High Humidity

At very high humidity (>80%R.H.), especially if the area to be coated is cooler than the warm and humid air, it is possible for the moisture in the air to condense on the surface. This is known as being at Dew Point or below, similar to the fogging of a mirror in a bathroom when showering.

A simple way to calculate the dew point is by applying the formula

Dewpoint (Temperature in °C) = ( Ambient Temperature ) - (100-RH%) / 5

In these situations, the coating can bloom, whiten (tiny bubbles), foam, lose gloss, or flow poorly, and be difficult to apply as it cures too quickly. Note that in cooler conditions, you may experience dew point issues, especially in foggy, damp conditions.


Coat in a controlled atmosphere, preferably inside a building. The materials in a building usually buffer the humidity, holding it more constant. For more details, refer to the following article from the Danish Musuem:

Very low humidity

At the other end of the humidity scale is the situation of very low humidity where it is possible for there to be not enough moisture in the air to cure the products at an acceptable rate.


Where the humidity is very low (<30% R.H.) consider hanging damp towels or drop-sheets nearby to raise the humidity, or perhaps put wet towels in the work area. If possible, use a small fan to circulate the moist air, taking care not to stir up too much dust.

Another remedy in the case of Duracoat MCR Gloss and MCL Gloss, is to use some Uroxsys Through Cure Catalyst. This actively helps the migration of moisture from the air and into the curing film. Use it up to the maximum level given in the product Technical Data Sheet. (5% for MCR Gloss, and 1.5% for MCL Gloss).

The use of Uroxsys Through Cure Catalyst is not recommended for the Uroxsys Marine Clearcoat Range, Satins or Matts.

Other Suggestions

You may like to consider buying a Hygrometer (humidity-level meter) from your hardware store for about $75 - $100. It is good practice to measure the temperature and moisture level on the job before the application of each coat, and to record it on a job sheet with the batch number of the product used. Once the humidity has been measured, the above guidelines can be used to decide how to manage the curing conditions.

Please note that the measured humidity level using one of the above hygrometers will be a guideline only and will not be scientifically verifiable. If in doubt, calibrate against saturated sodium chloride (table salt). For more details on measuring relative humidity, please follow the links below:


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