Best Practices: Tips for Specifying
And Applying Elastomeric Coatings
First of Two Parts
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.
Water-based elastomeric coatings are often the go-to choice for exterior concrete/masonry wall applications.
But they can be tricky to apply correctly and should be avoided for some applications—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 the 2-3 mil DFT of conventional paints).
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).
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 also be an attractive choice for specifiers or owners because of their durability. Many manufacturers offer a 10-year warranty, compared to 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.
Just be careful.
What to Watch For
Too Little Product Applied
All too often, we see elastomeric projects where the dry film thickness of the finished system is woefully lacking. The problem often stems from painters believing they can achieve the required dry film thickness in one coat.
However, the required wet film thickness for many elastomeric systems is 27-32 mils. It’s highly unlikely that an acceptable finish can be obtained by spraying or rolling a product to this WFT.
This is one coat of elastomeric applied at 18 mils DFT. Note the ropey, lumpy texture.
When painters try, the result is an exceedingly rough texture marked with roller sags and runs. This produces 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 (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 manufacturers’ product data sheets, 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 percentage and the time required between additional coats, but fail to specify the recommended wet film thickness per coat (for example, how thick a painter can apply this material without creating that “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, such as “60-65 square feet per gallon,” but this is an exceedingly sub-optimal way to specify the recommended DFT. We know few painters (or inspectors, for that matter) who convert that language into a usable guideline. 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. The final DFT, in turn, should be based on the manufacturer’s recommendations, which ideally are written plainly and clearly on the product data sheet.
Also note that this DFT must be maintained on the substrate's peaks and valleys. This is especially important when coating rough surfaces like stucco.
It's also 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).
Inexperience and Thinning
We see the “inadequate DFT” problem more frequently when the work is done by painters who are not experienced 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 gauge than thinning with solvent. You can add as much as a gallon of water to a five-gallon pail of product and not see much difference.
One easy way to gauge if an elastomeric coating was applied too thinly 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 3,000 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.
Ensuring Good Adhesion
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 before coating application.
Cracks and bugholes must be repaired and concrete dust and debris removed before elastomeric coatings are applied.
Care must also 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" (for example, 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 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 step unless it’s specified, so backrolling should be included in the project specifications.
Next Week: The Case of the Two Towers