How much is too much? Tips for repainting with elastomeric coatings

Based on the experience of PQA Inspector Dave Lick and MPI’s Level 2 Maintenance Repainting Coatings Specialist Course

In our experience, many architects and owners specify elastomeric coatings for exterior concrete and masonry walls as a reflex reaction; 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 elastomeric coatings? Can you just add another coat?

Elastomeric coatings are noteworthy for their elongation (aka stretchiness) that allows them to tolerate some substrate movement and 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).
  • they “breathe:” their permeability allows moisture vapor (water in its gaseous phase) to readily pass through the film without compromising the film’s 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 may 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, and eventually the multiple layers of elastomeric 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.

This begs the question: how much elastomeric is too much before you can apply another layer?
Unfortunately, since elastomeric coating product characteristics vary greatly between manufacturers, and “how much is too much?” depends on a wide variety of factors, there’s no industry-accepted rule prescribing what DFT of existing layers of coating is the “deal-breaker” such that adding 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.

So the approach for repainting elastomerics draws on the first rule of maintenance repainting: the specification writer (or their representative) must start with a thorough assessment of the existing coating.








Assessing the Condition of the Existing Coatings

Verifying the adhesion of the existing layers of coating is the first step. Elastomeric coatings are weightier than conventional latex, so if the adhesion of the existing coatings is already compromised, simply 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, keeping in mind that it’s not how many coats that are on the surface that matters, but instead 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.

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 from the surface, it can’t re-attach itself; it is forever delaminated.

In this inspector’s 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 the 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 determined to be sound. How can you reduce the risk?

When the Existing Layers are Sound but Thick

Most coating manufacturers will advise you to coat an elastomeric with another elastomeric. But if you fear the existing layers are teetering on the verge of impermeability, alternatives must be considered. This inspector has 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 that the elastomeric only stretches when it’s “asked” to – e.g. 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.

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, e.g. 5-6 mils DFT instead of the required 10-20 mils DFT. This may seem incongruous with what we preached in the previous 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 versus doing so with a repaint over many layers of existing coating. It is reasonable to assume that 40-50 mils of intact existing elastomeric will provide the required waterproofing and crack-bridging properties.

Compatibility of the Recoat Product

If you’ve determined it’s okay to repaint with another 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 properties but offer the potential drawback of only being able to be re-coated with themselves. So if you start with a silicone elastomer, it may be impossible to overcoat it with an acrylic elastomer. Note that silicone-modified acrylics can be re-coatable with conventional acrylic elastomeric coatings.

Overcoating Other Coating Types with Elastomerics

Latex. Sometimes rough surfaces such as stucco are initially coated with conventional latex that subsequently cracks in service. When it’s time for repainting, it may be appropriate to specify an elastomeric coating. However, if the latex-coated substrate is a smoother surface such as tilt-up concrete, caution is heartily 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 prior to 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 that were 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 to be sound and compatible with the new coating – but if there is any chalking visible on the surface, it must be removed prior to coating application. Note that MPI identifies two different types of clear water-repellent sealers: paintable (MPI #34) and non-paintable (MPI #117); so as long as the existing coating is of the MPI #34 type, it’s okay to overcoat with an elastomeric.

Unknown? When the project involves repainting a surface with many layers of existing coating but no record of the history of the 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 nor 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 the previous article, we discussed the option of spot repair if there are isolated areas of poor adhesion and blistering. But if multiple spot adhesion tests confirm that the coating film is unsound and blistering is pervasive across the surface, 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 prior to application of the new coating or the problem is going to recur.

For coating removal, in the inspector’s 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 as well as the flexible and resilient nature of elastomerics makes blasting slow and difficult.

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