The Case of the Two Towers: Tips for Specifying and Applying Elastomeric Coatings

Based on the experience of PQA Inspector Dave Lick and MPI’s Level 2 Architectural Coatings Specialist course

Waterbased elastomeric coatings are often the go-to choice for exterior concrete/masonry wall applications. But they can be tricky to apply correctly, and for some applications they should be avoided even if they appear to be the leading choice. In this article, we’ll begin a series describing best practices for working with elastomeric coatings and how to avoid costly problems.

Elastomeric coatings are higher volume solids (45-60%) than conventional paints, and are applied in films that typically attain a dry film thickness in the range of 10-20 mils per coat (versus conventional paints with a DFT of 2-3 mils). Initially designed as a waterproofing solution for stucco, which is a relatively rough surface that tends to crack, elastomerics are also well suited for tilt-up or poured-in-place concrete and masonry wall surfaces (including CMUs) because they provide excellent waterproofing properties; can tolerate some substrate movement; and their stretchiness (150-400% or greater elongation without breaking throughout their service life) allows them to fill or bridge even moving hairline cracks.

Elastomeric coatings can be an attractive choice for specifiers or owners because of their durability; many manufacturers offer a 10-year warranty versus the typical two-year warranty for conventional coatings. So it’s not unusual to see elastomeric technology specified for applications where a high quality exterior latex coating may actually be equally suitable.

What to Watch For… Too Little Product Applied

With elastomerics, all too often we see projects where the dry film thickness of the finished system is woefully lacking. The problem often stems from the painter believing he can achieve the required dry film thickness in one coat. However, the required wet film thickness for many elastomeric systems is 27-32 mils wet. It’s highly unlikely that an acceptable finish can be obtained by spraying or rolling a product to this WFT. When painters try, the result is an exceedingly rough texture marked with roller sags and runs, producing a ropey surface that one of our architects glumly referred to as a “lumpy-looking mess” when viewing a test patch applied to his one-coat specification.

So in our experience, multiple coats (and frequently more than two) are required to provide the level of waterproofing that elastomeric products were designed to achieve.

Vague Product Data Sheets

The misconception that one coat may be sufficient is not helped by many of the manufacturer’s product data sheets we see, which cheerfully state that “one to two coats” are required. In fact, we see many elastomeric coating product data sheets that are vague or void of some very useful data. For example, the data sheet may state the product’s % solids and the required time between applying additional coats, but will fail to specify the recommended wet film thickness per coat e.g. how thick a painter can apply this material without creating the afore-stated “lumpy-looking mess.”

Also, many product data sheets don’t state or make it difficult to find the manufacturer’s recommended minimum DFT for the finished system. Some may specify the theoretical coverage e.g. “60-65 square feet per gallon,” but this is an exceedingly sub-optimal way to specify the recommended DFT, as we know few painters (or inspectors, for that matter) who convert that language into a useable guideline — and besides, the roughness of the surface can alter the coverage.

So a good specification for elastomeric coating application should not state the number of coats to be applied, but rather the final dry film thickness of the system — which should in turn be based on the manufacturer’s recommendations, which ideally the manufacturer has written plainly and clearly on the product data sheet.

Also note that this DFT must be maintained on the peaks as well as the valleys of the substrate. This is especially important when coating rough surfaces like stucco, and is another reason why the specification should always require backrolling if the coating was applied by spray; otherwise, you may achieve the required 15 mils DFT in the low areas (valleys) of the surface, but not on the high areas (peaks).

Inexperienced Painters and Thinning

We see the “inadequate DFT” problem more frequently when the work is done by painters who are inexperienced at applying elastomeric coatings. Since elastomerics have a viscosity that’s drastically different from conventional paints, inexperienced painters may unwittingly decide to thin the product, especially if their spray equipment is not matched to its viscosity and stickiness. Furthermore, the end result of thinning with water is harder to gage than thinning with solvent; you can add as much as a gallon of water to a 5-gallon pail of product and not see much difference.

One easy way to gage if an elastomeric coating was applied too thin is to compare the gallons of material used to the number of square feet covered. Theoretical coverage with elastomerics is in the range of 60-100 square feet per gallon to yield the final dry film thickness, versus the typical 400 square feet per gallon attained with conventional paints. So if the paint manufacturer verifies that 10 gallons of material were used but the contractor shows you 3000 square feet coated, you know you’ve got a problem.

It is also noteworthy that elastomeric coatings are appreciably more expensive than conventional paints, so a less-than-honest painter will be tempted to apply less than the required film thickness.

Other Issues with Elastomerics

Good adhesion requires a clean, sound surface

Elastomerics achieve very little penetration of the substrate; they essentially lay on top of the surface. Consequently, the concrete finishing contractor must first repair any bigger-than-hairline cracks in the walls and fill any voids or bugholes prior to coating application, and care must be taken to properly rid the surface of any residual concrete or sacking dust or debris; otherwise, the coating film or adhesion may be compromised.

Primer: yea or nay?

To prevent issues with adhesion, some elastomerics are formulated to be applied over a ‘primer’ (e.g. conditioner/sealer); the manufacturer’s data sheet will state if one is required. Suitable concrete sealers/conditioners may be applied over a clean surface free of dirt, dust and debris; you’ll find some approved under MPI #3 Alkali-Resistant Primer. We find the clear non-pigmented products to perform best, as color pigments may hinder penetration.

In our experience, when applying elastomerics to concrete block, conventional block filler may be a sub-optimal primer choice. If any moisture enters the block through some flaw or defect in the wall, the elastomeric coating will trap and seal it inside, and in the moisture’s futile effort to escape to the exterior, it can soak the block filler and essentially turn it to mush.

Backrolling Sprayed Surfaces

If the elastomeric is spray applied, each coat must be backrolled before application of a subsequent coat. Many painters we know will not add this extra step unless it’s specified, so backrolling should be included in the project specifications.

The Case of the Two Towers

A GC/developer built two towers 36 stories high, and did not employ third party inspection during coating application. One building was already finished and occupied and the second one still in progress when the owner’s QA representative examined the occupied building and grew concerned that the DFT of the material was too low. How did he guess? Well, there’s a world of difference between the surface appearance of 5 mils DFT of elastomeric versus 15 mils DFT. 5 mils DFT of elastomeric coating looks much like a conventional latex application, with a mild roller stipple and the underlying texture of the substrate clearly visible beneath the coating. By contrast, a 15-mil DFT finish will have a thick, almost rubbery appearance, the outside edges may be slightly rounded, and the underlying surface texture will be substantially softer — in fact, if the initial substrate was fairly smooth, the substrate surface texture may disappear entirely.

And not only did the QA rep suspect the finish was applied too thinly, he also discovered variations in the level of adhesion. So he called in the paint inspector to take a look.

This inspector uses two methods to measure the DFT of an elastomeric coating on concrete. Ultrasound tools are non-destructive and can indicate the thickness of each individual coat; how many coats were applied (so long as they weren’t applied wet-on-wet); and to a certain extent, the profile/roughness of the underlying surface. His other option is a soft-jaw micrometer that measures the thickness of a small area peeled from the surface.

After measuring the thickness, the inspector measures adhesion with a crosscut tool in multiple random areas according to ASTM D3359 Method A, which is suitable for coating thicknesses greater than 5 mils. If adhesion is found to be lacking, the inspector will look for these telltale causes: an excessively smooth or shiny surface underneath the coating, or residual concrete or sacking dust clinging to the underside of the delaminated paint film. Both are evidence of poor surface preparation. With elastomeric adhesion issues, dust and debris are frequently the culprit: as stated earlier, any residue that remains on the surface prior to coating can substantially compromise coating adhesion.

Fixing the DFT

If the adhesion is adequate, the solution for achieving the correct film build is straightforward, though hardly convenient: the owner will face the aggravation of having the contractor recoat one tower that’s occupied and a second that will now be off schedule. Thankfully, elastomeric coatings have a generous re-coat window, so the delay between coating applications isn’t an issue; the contractor must powerwash the surface to remove any dust or debris that has collected, and then apply sufficient additional coats to attain the specified dry film thickness. The same coating manufacturer and product should be used (same on same) to keep the warranty intact.

Dealing with Adhesion Issues

However, when the adhesion is poor, the owner faces a more daunting and costly challenge. If the areas of bad adhesion are isolated, the failed sections can be scraped back to a sound surface and then power washed (2500 – 3500 psi) to assure no residual debris remains. Once the surface is dry, the painter must now achieve a non-visible touch-up, which is no easy task. To create a uniform surface, the new coats of elastomeric coating must be carefully applied to bring the thickness of the repaint area up to the same thickness as the surrounding area; this may require feathering back the edges of the adjacent intact sections. The whole wall may then be powerwashed again, and repainted edge to edge.

If the inspector found areas of poor adhesion all over the surface, all 36 floors of both towers would need to be stripped down to bare concrete and re-coated, which is a mighty costly and time-consuming endeavor. We’ll re-visit this in a second installment next month on issues with maintenance repainting with elastomeric coatings.