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Best Practices:
Winter Painting—Weather or Not (II)

Friday, September 5, 2014

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Second of Two Parts

The painting inspector's gut feeling, unfortunately, had proved spot-on.

As described in the first installment, the GC had decided that January and February of the brutal winter of 2013-14 was the time to paint the old big-box store for its transformation to a new purpose.

At the inspector's insistence, the job was contained and the containment controlled for temperature and humidity during the painting operation. Then, as soon as the second coat was dry, but before the 48-hour curing period, the containment was removed—against the inspector's recommendation.


The lower horizontal edge shows water trapped between the existing yellow latex coating and the new elastomeric topcoat, leading to countless blisters.

Then came the rains. And then the sunshine. And then the blisters. By the thousands.

Cause and Effect

The cause of the blisters was clear: The rain had pounded through the uncured elastomeric and became trapped between the new film and the intact latex underneath.

The problem was likely exacerbated by the fact that the architect had insisted on a black finish. Black cures more slowly than other colors.

Usually, this small delay in cure time is not a problem. But in this case, where the GC wanted the scaffolding/containment removed the day after painting, a faster cure time was required.

Also, black pigments absorb heat rather than reflecting it, so the sun's appearance and warmth expedited the process of turning the trapped rain droplets into their gaseous moisture vapor.

And since a gas has more volume than a liquid, it forced the stretchy film to delaminate and blister.

What to Do

Now, it’s true that the blisters would lose volume and flatten out when the wall cooled down again, but at that point, the coating in the blistered area has already quit. It has delaminated and will never adhere to the wall again.

And, as this inspector is fond of saying, you’re not going to fix it by looking at it.

So now, the GC is looking at a wall 300 feet long and 20 feet high that was meant to be a showcase backdrop for huge display banners to promote this new development—and it’s a virtual ocean of unsightly blisters. What to do?

The inspector suggested power-washing and scraping the surface to remove as many blisters as possible, and then recoating.

Water seeps out of broken blisters after the wall was hit by rain before curing.

If the wall substrate was smooth—like, say, tilt-up concrete—this could result in the wall looking like it has leopard spots, especially when the finish is a deep tone like black. But fortunately, stucco surfaces are rough, so the repainted areas would not be so obvious.

Round II

A wall the same size was scheduled to be painted after this one.

This time, the GC brought in hot-water heaters instead of propane, along with an electric fan to evenly distribute the heat, and he took great pains to secure the shrink-wrapped containment.

In addition, he kept the scaffolding and sheeting up a full week after the second coat of elastomeric coating was applied. After the scaffolding and containment were taken down, the walls were subjected to several days of pelting rain.

But this time, when the sun finally came out again, the new coating was completely intact.

The Moral of the Story

The paint supplier’s technical data sheet should specify the chosen product’s acceptable range for ambient (air) temperature, the temperature of the surface to be coated, the relative humidity, and the length of time the coating needs in order to cure properly before being put into service.

All too often, premature failures happen because coatings were applied when ambient or surface temperatures were too cold or too hot, the weather too humid, or some other environmental mismatch with the application recommendations.

(The paint inspector in this case had reviewed the sheet before application, confirming his misgivings about the GC's coating plan.)

Environmental Controls and Inspection

Containment and heating are certainly a viable strategy to “create your own coating weather.” So is the use of dehumidification equipment, when the season flips the other way and it’s excessively hot and humid.


The rains, followed by sun, produced a bumper crop of blisters.

The contractor can maintain the construction schedule while assuring coatings are applied under suitable environmental conditions by using atmospheric controls to maintain the specified range of ambient and surface temperatures as well as humidity.

But those controls add value only if they are executed properly. Shrinkwrap containment on a building doesn’t achieve its design goal if the containment comes loose from blasts of wintry winds that reduce the temperature inside to below freezing.

If an inspector had not been contracted to be on site to monitor the atmospheric conditions inside the faulty containment and insist the flaws be fixed, a premature failure might have already been in the works before the containment was removed.

And while we appreciate the GC installing heaters inside the contained area, using propane heaters in sealed areas can generate high humidity that may adversely affect the applied coating.

The better solution was used on the second wall: hot water heaters plus a fan to circulate the warmed air.

Interior Issues...

Maintaining proper environmental conditions is not limited to exterior coatings work.

During new construction, interior paint application is sometimes started before the heating system is operating and the building is completely closed to weather.

In mild seasons, conditions will be acceptable. In the winter, late fall and other times, the quality of the finished work may be severely compromised if proper attention is not given to the product’s requirements for surface temperature and ambient air temperature.

So controls such as fans, dehumidifiers, and heaters should be used when needed to control interior conditions.

...And a Warning

However, we don’t recommend using propane heaters inside sealed buildings.

The wall was properly wrapped and heated throughout the curing process. Despite four days of rain, there were no blisters.

As noted, these can adversely affect the cure and sheen of applied coatings, because of the high humidity generated when the fuel burns.

The problem is compounded if latex paints are specified, since latex creates additional moisture vapor as it dries.

That moisture vapor can then condense on cold surfaces such as windows, frames and exterior walls, and will adversely affect paint performance. Proper airflow is important to ensure drying of the paint and to reduce the build-up of fumes that can pose safety and health risks.

Ensure the Cure

Today may be a great day to paint; tomorrow, however, may not. Environmental conditions must remain within the specified limits throughout the drying or curing process, until the newly applied coating can withstand normal adverse weather.

So when paints need more than a day to cure, the weather forecast must be taken into consideration before painting begins.

If the uncured film encounters rain, excessive heat or frost, defects will result. Rain or dew on exterior surfaces during the drying process can lead to appearance, durability, and blistering problems. Latex coatings may wash off  if rained on during the early stages of drying.

Flat and eggshell finishes can show discolored spots where the water has made contact. Gloss and semi-gloss paints often show dulled gloss and discolored spots. Even alkyd and oil-based paints are not immune to the effects of rain, particularly during the early stages of drying.

And although it might seem obvious, when a product requires multiple days to cure—or the spec requires several coats over several days—the proper temperature and humidity must be maintained overnightas well, which means that heaters must run continuously.

Keeping Records

Inspectors should also monitor and record environmental conditions before and during paint application and during drying; these records should be saved and made accessible to the facility owner.

That data can be valuable should a failure occur down the road, or when planning maintenance painting operations.

Editor's Note: This article was written by PQA Inspector Dave Lick, incorporating MPI Level 2 Expert training, and is reprinted with permission from the MPI (Master Painters Institute) newsletter. MPI content describes best practices for commercial, institutional, and light industrial painting.


Tagged categories: Coating failure; Exterior Wall Coatings; Good Technical Practice; Master Painters Institute (MPI); Paint application

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