Weather Or Not: How good intentions (and flawed execution) can doom a winter painting project

Based on the experience of PQA Inspector Dave Lick and MPI Level 2 Expert training






In our experience, almost all the premature coating failures we see are for one of these reasons:

  1. The wrong coating system was specified for that substrate or service environment
  2. The painter failed to conduct proper surface preparation prior to paint application; or
  3. Coatings were applied during unacceptable environmental conditions

That third item might seem obvious to all us self-proclaimed “coatings experts” — but here’s a case that proves the steps required to apply coatings under proper environmental conditions aren’t always as straightforward as we’d like, especially when a GC’s schedule is pushing the envelope (This is why MPI’s Level 2B Maintenance Repainting course begins with a detailed review of how to manage environmental conditions during painting operations).

The Case of the Big Box Walls

This inspector was recently assigned to a renovation project where a large big box store was to be re-purposed into a multi-use development. The exterior substrate was mostly acrylic (synthetic) stucco Exterior Insulation Finishing System (EIFS), painted with an off-white cream latex that had performed well over its 20-year life.

The initial inspection found that the EIFS was in good shape and the latex finish was intact and tightly adhering, albeit a bit dirty. This would have been an easy repaint job — a powerwash to remove dirt and contaminants followed by two finish coats— if it was conducted during mild weather conditions.

But as happens far too often around here, the GC had scheduled the coating work for January/February. Now, we realize that a GC on a project located in, say, Syracuse or Toronto would never tempt fate with that timing, but in our Pacific Northwest climate, it’s plausible to have milder dry weather suitable for exterior painting some weeks during the winter months. It’s just not likely.

The winter of our discontent

This cycle went on for more than a month. When the GC told the paint inspector it was time to paint, the inspector said absolutely not; the environmental conditions were not suitable. So the GC offered to contain the area with shrink-wrapped scaffolding and propane heaters in order to maintain the wall surfaces at a temperature suitable for painting. The inspector warily agreed.

The scaffolding and containment were set up, and the contractor started by powerwashing the surface to remove visible dust and contaminants. The surface was then allowed to dry over several days. Throughout this process, the inspector took regular readings to check that the temperature inside the containment was acceptable for painting, and was dismayed to find inconsistent results. The shrink wrap was not completely secured; when high winds stripped off portions and overpowered the propane heaters, ambient air temperature readings inside this purportedly heated and contained area would range from 12° C (53° F) down to -1 or -2° C (30 – 28° F).

The inspector then demanded the containment be shored up until a consistent suitable interior temperature could be maintained. Painting work started on an east elevation. The contractor sprayed and backrolled the specified black elastomeric finish in one day and applied the second coat the next day. Throughout painting operations, the temperature and relative humidity inside the containment was monitored to verify that the product was being applied in conditions that were within the specified range for the product.

What a difference a day makes

The day after the second coat was applied, mindful of the daily rate he was paying for it, the GC removed the scaffolding and containment from the walls. This was against the recommendation of the inspector, who knew from the product data sheet that a minimum 48 hours of cure time was required before the elastomeric finish could withstand atmospheric service — and even longer before it would develop full elastomeric properties. That wall surface should have remained contained and heated for at least one more day; otherwise, the only hope was to pray for mild weather.

Alas, consistent with the construction version of Murphy’s Law, the day after the second coat was applied, the now-exposed wall got pounded with torrential rains. When the sun came out the subsequent day, lo and behold the newly painted wall was littered with countless (thousands?) of blisters. They bulged under the stretchy elastomeric finish and as the day went on, they grew with each added degree of warmth. A cut into the black blisters caused telltale water to pour out.

Cause and effect

The cause of the blisters was clear: the rain had pounded through the un-cured elastomeric and become trapped between the new film and the intact white 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 coming out and warming the building expedited turning the trapped rain droplets into their gaseous moisture vapor stage; since a gas has more volume than a liquid, it forced the stretchy film to delaminate and blister.



What to do now

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’s 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 to hang display banners to promote this new multi-purpose development — and it’s a virtual ocean of unsightly blisters. What to do? The inspector suggested the surface be power washed and scraped to remove as many blisters as possible and then recoated. 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.

A second wall the same size was scheduled to be painted after this first wall. 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 shrinkwrapped containment. Furthermore, he kept the scaffolding and sheeting up a full week after the second coat of elastomeric coating was applied. After the scaffolding and containment was taken down, the walls were subjected to 3 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, too hot, relative humidity was too high, etc.

Using environmental controls…and inspection

Containment and heating are certainly a viable strategy to “create your own coating weather” — in fact, so is the use of dehumidification equipment when the season flips the other way and it’s excessively hot and humid. The contractor can maintain the construction schedule while assuring coatings are applied under suitable environmental conditions by utilizing atmospheric controls to maintain the specified range of ambient and surface temperatures as well as humidity.

But those controls only add value if they are executed properly. Shrinkwrap containment on the building doesn’t achieve its design goal if the containment comes loose from blasts of wintry winds that reduce the temperature inside the containment 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 preferable solution was used on the second wall: hot water heaters plus a fan to circulate the warmed air.

Environmental Controls for Interior Paint Application

The importance of maintaining proper environmental conditions is not limited to exterior coatings work. During construction of new buildings, interior paint application is sometimes started before the heating system is operating and the building is completely closed to exterior weather. In mild seasons, conditions will be acceptable; at other times (winter, late fall, etc.) the quality of the finished work may be severely compromised if proper attention is not given to meet the product’s requirements for surface temperature, ambient air temperature, etc.

So controls such as fans, dehumidifiers, and heaters should be used when needed to control interior conditions. However, we don’t recommend the use of propane heaters inside sealed buildings; as mentioned above, 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. The moisture vapor can 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.

Maintaining proper conditions throughout the cure process

Today may be a great day to paint; tomorrow, however, may not. Environmental conditions must be within the specified limits throughout the drying or curing process, until the newly applied coating can withstand ‘normal’ adverse weather conditions. So when paints need more than a day to cure, the weather forecast must be taken into consideration before painting work commences.

If the uncured film encounters weather conditions such as rain, excessive heat, or frost, defects or poor physical paint properties will result such as happened on this ill-fated wall. Rain or the formation of dew on exterior surfaces during the drying process can lead to appearance, durability, and blistering problems; latex coatings can be washed off the surface 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 applying several coats over several days — the proper temperature and humidity must be maintained overnight as well, which means the heaters must run continuously.

The Importance of record keeping

Inspectors should monitor and record environmental conditions before and during paint application, and during drying, and these records should be saved and accessible to the facility owner. The data contained within can be very valuable should a failure occur down the road, or when planning maintenance painting operations.

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