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Best Practices: How Much is Too Much?
Tips for Repainting with Elastomerics

Monday, November 25, 2013

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In our experience, many architects and owners specify elastomeric coatings for exterior concrete and masonry walls as a reflex. They’ve used elastomerics successfully on previous projects and assume they’re the best choice again.

But what if it’s a repaint job and the surface already has multiple layers of elastomerics? Can you just add another coat?

Elastomeric Repainting
Photos: MPI

Got blisters? Too many coats of elastomeric can tip the coating from a breather to a water-trapping barrier.

Elastomeric coatings are noteworthy for the elongation (aka stretchiness) that allows them to tolerate some substrate movement and to bridge moving hairline cracks.

But they’re also attractive for these two seemingly competing virtues:

  • They’re a virtual barrier to water in its liquid phase, so they’re excellent for waterproofing (you’re more or less wrapping a building in plastic); and
  • They breathe. Their permeability allows moisture vapor to readily pass through the film without compromising its adhesion to the surface.

The Need to Breathe

A product’s breathability is measured in perms, defined as the number of grains of water vapor per hour per square foot per inch of mercury-water vapor pressure difference. A “breathable” coating is generally considered one that is 10 perms or greater, and typical elastomeric coatings range from 8-12 perms or more.

So an initial application of elastomeric applied at the proper DFT fulfills the specifier/owner’s requirement for a waterproof, but breathable, coating.

But with each additional repaint, the permeability of the existing film is reduced; eventually, the multiple layers  become a barrier, instead of a breather.

Water vapor that can no longer be breathed out of the wall gathers behind the coating and condenses into water. That water cannot escape (remember, elastomeric is uber-waterproof), so it forces the paint film to delaminate, and the film stretches into large water-filled blisters right down to the bare surface.

It’s not uncommon to break these blisters and see water pour out.

Which Begs the Question...

How much elastomeric is too much before you can apply another layer?

Unfortunately, elastomeric coating characteristics vary greatly among manufacturers, and “how much is too much?” depends on a wide variety of factors.

So there’s no industry-accepted rule prescribing what DFT of existing layers is the “deal breaker” that means another X DFT of elastomeric will reduce the perm rating to the point that water vapor can no longer pass through.

The manufacturer’s data sheet might specify the required DFT for one application, but it won’t state a maximum allowable DFT that includes existing layers of old coating.

Therefore, the approach for repainting elastomerics draws on the First Rule of Maintenance Repainting: The specification writer (or his or her representative) must start with a thorough assessment of the existing coating.

Assessing the Condition of the Coatings

Verifying the adhesion of the existing coating layers is the first step.

Soft-Jaw-Micrometer

A soft-jaw micrometer measures this elastomeric coating film at 41 mils DFT.

Elastomeric coatings are weightier than conventional latex, so if the coating adhesion is already compromised, adding another heavy layer of material will only exacerbate the problem.

After adhesion testing, the inspector takes samples from random areas across the surface and uses a soft-jaw micrometer to measure the DFT. (Keep in mind that it’s not how many coats on the surface that matters, but  the total DFT.)

At 30+ mils DFT, the inspector will look carefully for telltale signs of breathability/vapor migration issues, starting with water-filled blisters.

Behind the Blisters

Note that blisters may be less evident or appear smaller in the morning when it’s cool. As air temperatures inside and outside fluctuate, water vapor that’s trapped in walls behind a non-breathing film goes through a continuous cycle of condensing into water, expanding, stretching, and delaminating the film.

As the sun hits the wall and heats up the surface, the blisters will expand and become more visible. So best practice is to inspect the surface in mid-afternoon while the wall is in direct sunlight and to check the Southern exposure instead of the Northern exposure.

And even though blisters in an elastomeric film can go from big to small, once the paint film has blistered and delaminated, it can’t re-attach itself; it is forever delaminated.

In our experience, if the existing surface has two or three layers of elastomeric coating that are intact with no evidence of blistering, and adhesion is verified to be adequate, it will be safe to repaint with another layer of elastomeric (especially because we so often find insufficient DFT was applied in the first place, as was described in a previous article).

But when a surface already holds 50 mils of elastomeric coating, applying additional coats is a calculated risk, even if the adhesion of the existing system is deemed sound.

How can you reduce the risk?

When Existing Layers are Sound but Thick

Most coating manufacturers will advise you to coat an elastomeric with another elastomeric.

But if the existing layers appear to be teetering on the verge of impermeability, alternatives must be considered. We have been on projects where instead of re-coating with an elastomeric, the existing layers were successfully re-coated with a conventional satin latex, which adds only 2 mils of DFT to the system.

While the satin latex does not offer the 3x or 4x elongation of the underlying elastomeric, conventional latex does have some flexibility.

And remember: The elastomeric stretches only when it’s “asked” to (for example, when the building moves). If the building has been in place for years and substrate movement and new hairline cracks are no longer an issue, topcoating with conventional latex can be appropriate.

When considering this option, know that doing a test patch will not help, because the evidence required to validate this approach won’t surface in just a few days.

Option B

If the goal is to freshen up the surface with the same or a slightly different color (this won’t work if a drastic color change is required), another alternative is to apply a thinner film of elastomeric (say, 5-6 mils DFT instead of the required 10-20 mils).

This may seem incongruous with what we preached in that earlier article—that too often, elastomerics are applied too thin to achieve their design purpose—but there’s a world of difference between applying a starved layer of elastomeric in an initial paint job and repainting over many layers.

In these cases, it is reasonable to assume that 40-50 mils of intact elastomeric will provide the required waterproofing and crack-bridging properties.

Recoat Compatibility

If you’ve determined it’s OK to add a coat of elastomeric, you need to verify that the new coating is compatible with the existing system.

Elastomerics can be based on acrylic, acrylic blend, silicone, or polyurethane technology. Pure silicone-based elastomeric coatings offer tremendous waterproofing but can also be re-coated only with themselves.

So if you start with a silicone elastomer, it may be impossible to overcoat it with an acrylic. Note that silicone-modified acrylics can be recoated with conventional acrylic elastomeric coatings.

Overcoating Other Coating Types

Latex. Sometimes rough surfaces like stucco are initially coated with conventional latex that later cracks in service. When it’s time for repainting, it may be appropriate to specify an elastomeric.

However, if the latex-coated substrate is a smoother surface such as tilt-up concrete, caution is advised. You need to be very conscious of the integrity of the underlying latex.

If the adhesion is excellent, and the underlying concrete was sufficiently roughened before initial coating application, and there are no signs of excessive moisture behind the latex, it may be safe to proceed.

But if the adhesion of the latex appears to be compromised in any way, applying a thick, heavy layer of elastomeric over it can lead to fairly immediate failure.

We’ve seen the new elastomeric peel off in sheets right down to the surface.

Concrete Stains or Clear Sealers. Sometimes, we encounter buildings previously coated with concrete stains or sealers, and the specifier wants to re-paint with an elastomeric. This is a viable choice, so long as the existing coating is verified as sound and compatible with the new coating.

But if there is any chalking visible, it must be removed before coating application.

Elastomeric painting blister

A water-filled blister on a wall is a sign of too many layers of elastomeric coating. Complete removal will be required.

Note that MPI identifies two types of clear water-repellent sealers: paintable (MPI #34) and non-paintable (MPI #117). So long as the existing coating is of the MPI #34 type, it’s OK to overcoat with an elastomeric.

Unknown? When the project involves repainting a surface with many coating layers but no record of the previous work, every effort must be made to identify the existing coatings.

If identification is not possible or feasible, applying an elastomeric coating may be a risk that’s not worth taking. The best inspector’s spot-adhesion checks can’t validate the film integrity over the entire building or the quality of the underlying surface preparation, so using elastomerics could either create a problem when there wasn’t one, or make a small problem bigger.

Instead, conventional latex may be your best bet.

When Complete Coating Removal is Required

In a previous article, we discussed the option of spot repair for isolated areas of poor adhesion and blistering. But if multiple spot-adhesion tests confirm that the coating film is unsound and blistering pervasive, complete removal will be required.

The first step is to determine why the blisters formed. If there are construction defects or flaws allowing the ingress of moisture, these must be rectified before applying the new coating or the problem will recur.

For coating removal, in our experience, power tools, grinders, and scrapers are a better choice than pressure washing.

The higher pressures that may be needed to efficiently remove the coating can blow off the underlying sacking and damage the concrete, and scrapers are more effective at getting under the coating film.

Abrasive blasting is a poor option: Environmental restrictions and the flexible and resilient nature of elastomerics make blasting slow and difficult.

Editor's Note: This article was written by PQA Inspector Dave Lick 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: Concrete coatings and treatments; Elastomeric coatings; Good Technical Practice; Maintenance coating work; Masonry coatings; Master Painters Institute (MPI); Paint application; Topcoats

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