Query: Why is surface preparation (blasting) not allowed during night hours

[mnthiraviam]

Query: Why is surface preparation (blasting) not allowed during night hours

[nguyencz]

Have you control the color of surface for quality (visual inspection) and also due to at nigh you blasting in dew so getting a light rust.

[Han Ah kwang]

Surface preparation and the application of coatings and linings should be performed under optimum environmental conditions to help prevent failures. A variety of instruments are available to measure the five conditions that should be observed and tracked:

Air temperature
Surface temperature
Relative humidity (RH)
Dew point temperature
The difference between the surface and dew point temperatures

It is commonly known that most coatings will not dry properly at low temperatures and high RH. Less understood is the impact surface moisture has on the life and performance of materials.

Moisture forms on a surface when warmer, moist air comes into contact with it -- a process called condensation. Moisture will cause unprotected steel to rust. Trapped between a coating and a substrate, moisture will likely cause the applied system to fail prematurely.

Light condensation on blasted surfaces can be difficult to observe. Rather than detect this moisture, instruments are used to help assess the risk of moisture forming in the first place. Tests should be performed to calculate the dew-point temperature before, during and after the coating process. Dew-point temperature should be compared to the surface temperature to ensure the two are far enough apart that moisture formation is unlikely.

Careful observation of atmospheric conditions and a good understanding of their impact on the quality and long term health of coating and lining applications are important to all contractors and inspectors.

Environmental Conditions

AIR AND SURFACE TEMPERATURES

The first parameters necessary to assess the risk of moisture formation on a substrate are the temperature of the surface to be prepared or coated and the temperature of the air near that surface. At night, steel work usually radiates heat and is cooled below air temperature. During the day, it absorbs heat and is usually warmer than the air temperature.

Since surface temperature is often different from air temperature especially for work performed outside, both temperatures should be measured to avoid application problems should air or steel temperatures become too hot or too cold for satisfactory film formation. Application at incorrect temperatures can cause defects such as: blistering, pinholing, cratering, dry spray & mud -----ing. The coating manufacturer should specify the maximum and minimum surface temperatures for applying a coating.

ASTM D3276, “Standard Guide for Painting Inspectors (Metal Substrates),”1 states that the minimum surface temperature for coating application is usually 40ºF (5ºC). It may be as low as 0ºF (–18ºC) for “cold-curing” one or two-component systems or 50ºF (10ºC) for conventional two-component systems. Paint specifications may further state that painting should not be undertaken when the temperature is dropping and within 5ºF (3ºC) of the lower limit.

The maximum surface temperature for coating application is typically 125ºF (50ºC) unless clearly specified otherwise. A surface that is too hot may cause the coating solvents to evaporate so fast that application is difficult, blistering takes place, or a porous film results.



RELATIVE HUMIDITY

Cure rates are directly affected by RH -- the amount of moisture in the air expressed as a percentage of the total amount (saturation) possible at a given temperature. Moisture-laden air cannot hold as much solvent as dry air. Therefore, high RH can retard the rate of solvent evaporation. For this reason, the maximum RH at which coatings or linings can be applied and cured is generally set at 85%. Some coatings, however, require moisture to cure. Therefore it is important to check the specifications of the coating.

DEW-POINT TEMPERATURE

The dew-point temperature is the temperature at which moisture will begin to form on a steel surface. It is the temperature to which a volume of air must be cooled in order to reach saturation. It is a function of air temperature and the RH.

DELTA (DIFFERENCE) BETWEEN DEW-POINT AND SURFACE TEMPERATURES

The final parameter to note is the amount of separation between the surface temperature and the dew-point temperature. Moisture will likely form if they are the same. Even if they are close, the risk of moisture forming may be unacceptably high. Documents such as ASTM D3276 and the international standard ISO 8502-42 state that the surface temperature must be a minimum of 5ºF (3ºC) above the dew-point temperature during the critical 3 phases of coating: preparation, application and cure. This minimum separation also helps allow for surface temperature reduction as solvents evaporate or when cold coating materials are applied.

[marwanab]

Relative humidity is higher at night and coatings won't dry properly and also rust forms. Try and you will see my friend.

[gs153]

Please upload ASTM D3276-15 Standard Guide for Painting Inspectors. thanks