A coating is applied only when the surface temperature is above the dew point by the margin set in the specification, when relative humidity and temperature fall within the data sheet limits, and when those conditions hold until the end of the cure. In a tank or confined space, this is achieved with a dehumidifier, most often a desiccant unit, and indirect heating that introduces neither combustion gases nor moisture.
In Quebec and Ontario, a large share of coating work is done during shutdowns, often in spring, fall or winter, when outside air is cold and damp. Climate control then becomes as important as surface preparation. This guide is for maintenance managers and project managers planning the coating of tanks, water towers, vessels and pipes. It complements our tank and water tower painting page and our reference page on protective coatings.
Why does the dew point decide whether a coating succeeds?
Because a surface colder than the dew point picks up condensation, often invisible, and no coating bonds properly to a film of water. On freshly blasted steel, that moisture also causes flash rust almost immediately.
The dew point is the temperature at which ambient air, at its current humidity, starts to condense. It depends on air temperature and relative humidity. What matters is not the air temperature but the surface temperature: a tank wall in contact with cold ground or cold water can stay below the dew point while the air seems dry. ISO 8502-4 describes how to estimate the likelihood of condensation before application.
Conditions also change during the day. A surface that is acceptable at noon can drop below the dew point in the evening, when the air cools and humidity rises, or when cold product is pumped into a neighbouring tank. That is why a single morning reading is not enough.
Our article on flash rust shows the direct consequences of a surface that is too cold after blasting.
Which readings are needed, and when?
Measure air temperature, relative humidity, dew point and surface temperature before blasting, before each coat and at regular intervals during application and cure. Readings are recorded with the time and location.
| Parameter | What it tells you | Common instrument | Requirement |
|---|---|---|---|
| Air temperature | General working and curing conditions | Thermometer or electronic probe | Range on the data sheet |
| Relative humidity | Amount of water vapour in the air | Psychrometer (ASTM E337) or electronic hygrometer | Maximum on the data sheet |
| Dew point | Condensation threshold | Calculated from the two readings above | Basis for the margin |
| Surface temperature | Actual condensation risk | Contact thermometer | Above the dew point by the specification margin |
| Product temperature | Viscosity and application | Thermometer | Range on the data sheet |
Readings are taken where the coating is applied: at the floor, on the shell and under the roof of a tank, conditions vary widely. A data logger placed in the work area also provides a useful history if a defect appears later.
Dehumidifier or heater: which one is needed?
Often both. The dehumidifier lowers the dew point of the air, and the heater keeps the surface and the product within the data sheet range. Heating alone lowers relative humidity, but steel warms more slowly than air and can stay cold.
| Equipment | Principle | Strengths | Limits |
|---|---|---|---|
| Desiccant dehumidifier | An absorbent material captures water vapour | Effective even in cold weather | Needs an energy source to regenerate the desiccant |
| Refrigerant dehumidifier | Air is cooled to condense water | Simple in mild climates | Loses efficiency when the air is cold |
| Indirect-fired heater | The flame or element is isolated from the supply air | Clean, dry air with no combustion gases | Flue gases must be exhausted outdoors |
| Direct-fired heater | The flame heats the supply air directly | Fast temperature rise | Adds moisture and combustion gases: avoid in the work space |
The joint NACE 6A192 / SSPC-TR 3 guide covers dehumidification and temperature control during surface preparation, application and curing of coatings in tanks and enclosed spaces. It helps size and position the equipment.
Why avoid direct-fired heating?
Because a direct-fired burner releases water vapour and carbon dioxide into the air, along with carbon monoxide. The added moisture raises the risk of condensation, and some amine-cured epoxies react with moisture and carbon dioxide to form a greasy surface film, known as amine blush, that impairs the adhesion of the next coat.
Indirect heating separates combustion from the supply air through a heat exchanger, and the flue gases are vented outdoors. Electric, steam or glycol heaters are other options when that energy is available on site. Our article on cold-weather epoxy application during a winter shutdown details how cold affects epoxy cure.
Which precautions apply in a confined space?
In a tank or vessel, climate control goes hand in hand with safety: ventilation sufficient for solvent vapours, atmospheric monitoring, and an entry and rescue plan. Quebec and Ontario confined space rules, along with CSA Z1006, govern this work.
- Ventilation and solvent vapours. Ventilation keeps vapour concentration well below the lower explosive limit and the exposure limits. It is calculated from the product, the quantity applied and the volume.
- Atmospheric monitoring. Oxygen, flammable gases and contaminants are measured before entry and continuously during the work.
- Balancing ventilation and dehumidification. Humid outside air brought in by ventilation must pass through the dehumidifier, otherwise it cancels its effect.
- Suitable equipment. Electrical equipment and lighting matched to the area classification when flammable vapours are possible.
- Emergency plan. An attendant outside, communication and rescue means in place before entry.
Why hold conditions through the full cure?
Because a coating is not finished when it is dry to the touch: it keeps curing for the time stated on the data sheet, and that time depends on temperature. Shutting off heat or dehumidification too early exposes the film to condensation, incomplete cure and defects that only show up in service.
- Before return to service. Immersion linings, especially in potable water tanks, require full cure before filling, verified according to the data sheet.
- Between coats. The recoat window varies with temperature; our article on pot life and recoat windows explains how to read it.
- On concrete. Moisture in the substrate matters as much as moisture in the air, as our article on concrete moisture before coating shows.
The blistering, blushing and loss of adhesion that follow a poorly controlled cure are described in our guide to coating failure modes.
Frequently asked questions
Can you blast in humid weather if the coating is applied later?
Only if the surface stays dry and clean until application. Without dehumidification, blasted steel rusts quickly and must be blasted again. Conditions are therefore kept under control between blasting and the first coat.
Should the dehumidifier run overnight?
Yes, as long as the blasted surface or the curing coating can be exposed to condensation. Overnight shutdowns of the equipment are a common cause of flash rust and cure defects.
What margin above the dew point must be respected?
The one set in the project specification and the product data sheet. It applies to the surface temperature, measured where the coating is applied.
Who supplies and monitors the climate control equipment on site?
The specification should say. It should name who is responsible for the equipment, its power supply, continuous monitoring and the records, otherwise conditions may not be held during the cure.
How do you plan the work and get a quote?
Our crews and qualified subcontractors build dehumidification, indirect heating and condition logging into tank coating work, with the coordination provided by our project management service, including in Saguenay-Lac-Saint-Jean, the Côte-Nord and Ontario. Our article on tank lining shutdown planning helps prepare the schedule, and our industrial tank lining selection guide helps choose the system. Send dimensions, access, the planned period and the energy available on site through the request a quote form: pricing on consultation, reply within 24 business hours.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.