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April 16 - April 22, 2012

How long can a blasted surface be left before priming under different temperatures/relative humidity environments?



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Selected Answers

From remko tas of Futuro SRL on April 17, 2012:
     As a curiosity; flash rust did not occur for 10 days in a dry climate at an altitude of 4000m (13,000ft) in Bolivia, hundreds of kilometers away from the sea influence and contaminating industry. We could still safely paint the interior of the tank in one shot, achieving a high efficiency.

From Lubomir Jancovic of MSPLUB Inc. on May 1, 2012:
You have to prime the blasted surface of steel in up to 8 hours. Otherwise,  blasting was for nothing. If humidity is more like 60%, you have to prime the surface in up to four hours.

From Charles Williams of kurbtblast on April 16, 2012:
     Depending on the situation and coating system, we do not leave surfaces uncoated for more than 24 hours. Some epoxies have a tolerance level for flash rust (bloom), but when a zinc primer is required, we will coat the same day. Not all situations require same day priming, but some do. Our max time for leaving anything uncoated is 24 hours on warm dry days.

From Lee Edelman of CW Technical Service on April 19, 2012:

Always attempt to prime the prepared surfaces prior to the specified surface prep degrading. Most specifications will address this. Humidity and dew point should be monitored throughout the process, so that  when humidity or dew points exceed specification, there should not be any painting activities.

If the surface prep has degraded, most specifications will require reblasting to the specified surface prep.

From richard d souza of stoncor middle east llc on April 16, 2012:
     It is not about how long but the highest criteria is the steel temperature shall always be at least 3 degrees C higher than calculated dew point temperature. This margin of saftey is sufficient for all type of coatings. The common scenario is that the contractor blasts all day long, then experiences problems with the compressor or other piece of equipment, and lets the newly blasted steel sit a long time before coating. But the standard requires that the surface to painted must meet the cleanliness criteria set by the client or consultant which supersedes every other judgment. In Jordan, for example, oxidation or orange rust may not occur for serveral hours or days, but in UAE, you can probably expect to see rust breakthrough in few hours depending on the time of the day or night. It is almost impossible to state the effect of time,   temperature and humidity  on the blasted surface. This  must be the sole responsibility of the inspector, who conducts on-site checks of the following: -Place of work -Air temp min/max -Relative humidity -Dew point temperature -Steel temperature -Necessary ventilation -Type of weather-sun, wind etc. -Wind directions. And this must be done a minimum of thrice per shift. The inspector should have the sole right to determine the time without deviating from the specification.

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Tagged categories: Abrasive blasting; Humidity and moisture; Substrates; Surface preparation


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