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Best Practices: The Case of the 2 Towers

Wednesday, November 6, 2013

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Second of Two Parts

Last week, we tipped you to secrets for specifying and applying water-based elastomeric coatings—often, the go-to choice for exterior concrete/masonry wall applications.

This week, we'll show you the real-life consequences of short-cutting best practices for working with these super-durable, flexible, waterproof—yet tricky—products.

QA Questions

A general contractor/developer built two towers 36 stories high, but 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 Quality Assurance representative examined the occupied building and grew concerned that the dry film thickness (DFT) of the material was too low.

Elastomeric Coating
 

A crosscut adhesion test performed according to ASTM D3359 Method A reveals concrete debris and loose sacking under the paint film.

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.

Five 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, the substrate surface texture may disappear entirely if the initial substrate was fairly smooth.

Enter the Inspector

Not only did the QA rep suspect the finish was applied too thinly, but 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.

Seeking a Culprit

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.

Elastomeric Coating

Cracks and bugholes must be repaired and concrete dust and debris removed before elastomeric coatings are applied to ensure proper adhesion.

Both are evidence of poor surface preparation.

With elastomeric adhesion issues, dust and debris are frequently the culprit. As we warned in "Tips for Specifying and Applying Elastomeric Coatings," any residue that remains on the surface before 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 power wash the surface to remove any dust or debris that has collected, and then apply sufficient additional coats to attain the specified dry film thickness.

Elastomeric Coating

Five mils DFT of elastomeric looks much like a conventional latex application. This one coat applied at 18 mils DFT, on the other hand, has a ropey, lumpy texture.

The same coating manufacturer and product should be used (same on same) to keep the warranty intact.

Dealing with Adhesion Issues

When the adhesion is poor, however, 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. (See, for example, "The Un-touch-upables: Tips for Effective Touch-up.")

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.

But What If ... ?

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 future installment on issues with maintenance repainting with elastomeric coatings.

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: Adhesion; Coating failure; Coating inspection; Dry Film Thickness (DFT); Elastomeric coatings; Good Technical Practice; Master Painters Institute (MPI); Paint application; Specifiers; Wet film thickness

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