No good deed goes unpunished: The Case of the Laboratory Block Walls

From the experience of PQA Inspector Dave Lick

Much has been written about issues with coating exterior CMU walls — but here’s a case involving interior concrete block. File this one under the “no good deed goes unpunished” category: what seemed like a creative and well-founded solution can cause a much bigger problem down the road if all the pieces of the puzzle (e.g. the many ways walls can make paint fail) aren’t considered.

Two years ago, a shiny new laboratory was constructed with concrete block (CMU) interior walls. The architect had consulted his MPI Architectural Painting Specification Manual, which describes the recommended coating system options for 25 different substrates, and under Division 4.2 Concrete Masonry Units, Interior, he selected system INT 4.2F: “Epoxy, (tile-like) over latex block filler, for dry environments.” The first coat in the system is latex block filler MPI #4, followed by two coats of solvent-based gloss epoxy MPI #77.

In retrospect, that choice was a mistake. This was a “clean room” laboratory that would be subjected regularly to rigorous disinfection by being fumigated with a fog of hydrogen peroxide. If a qualified paint inspector had been consulted for this job, he would have taken note of that and advised the architect to instead choose MPI system INT 4.2G “Epoxy (“tile-like”) for wet environments,” for which the first coat is epoxy block filler MPI #116 instead of latex, followed by the two coats of the same solvent-based gloss epoxy MPI #77.

But that’s not the error that ultimately led to the dramatic failure that followed.

Holey Concrete Block

When the walls were ready for coating, the architect didn’t like the look of the block; it was too porous. We see this problem too often when concrete block is the construction material of choice: different density blocks have very different appearances. Lightweight block has larger, more numerous holes, while standard density block is “tighter” with fewer and smaller holes. So lightweight block may not have the desired appearance for a wall where aesthetics are an issue.

Furthermore, if a wall is constructed using different density blocks, the difference will be noticeable — and when the wall is an interior surface in a shiny new lab that is meant to be a veritable showcase with a smooth, hygienic finish, the uneven appearance and texture will be unacceptable. For projects where CMU walls must be aesthetically appealing, we recommend that the specifier and GC take the required steps to assure that that different density blocks NOT be intermingled on the same wall because no matter how much paint or block filler you apply, you’re going to see the differences in the block.

So how could the architect solve this problem and achieve the uniform surface the owner wanted? He had the blocks sacked. Sacking is a common practice for rendering a more uniform appearance for concrete walls, but in our experience, it’s not done often on block. Sacking is also difficult to do well; we’ve seen many a sacked wall where the end result was poorly-adhering material that readily delaminated from the surface, taking the coating system with it.

However, for these walls, the sacking work was done extraordinarily well and the finished blocks looked relatively smooth and uniform, light years ahead of what the walls looked like originally.

The specified system (latex block filler followed by two coats of epoxy) was then applied.

One Year Later…

Just over one year later, cracks formed in the coating and the edges started to lift a bit. The inspector was called in, put a knife to the lifted edges, and lo and behold, both the block filler and the epoxy topcoat readily came off in sheets right down to the sacking. We call that “shearing” failure: applying stress causes intercoat delamination, and the epoxy topcoat readily separates from the coat of filler beneath it.

There were numerous loose areas, and sheets of coating readily came off each time the inspector put his knife to a lifted edge.

So what happened?

It’s Called “Block Filler” for a Reason

There’s a reason this category of products is called “block filler” and not “block primer.” A true primer’s function is to (a) stick to the substrate, and (b) provide a good base for the topcoat to adhere to. On the other hand, a block filler’s job is to fill the holes in the block, and its adhesion is based on it being applied into a rough and porous surface. If it simply lays on top of a surface, it will have very poor adhesion.

We’ve seen problems like this before, where concrete block is part of or connected to a tilt-up or other type of concrete wall. The contractor takes the block filler he was applying to the block and also applies it to the wall, and finds out too late that block filler will not stick to concrete: after topcoating there’s no adhesion between the block filler and the wall, and the block filler will shear off the wall, taking the topcoat with it.

Knowing this, the paint inspector in the lab observed the sheets of delaminated coating and deduced: once the porous concrete block surface was filled with a good coat of sacking, (a) the need for applying a block filler was eliminated, and (b) so were any chances for the block filler to adhere to the surface. Furthermore, even though everything looked fine after the initial coating application, topcoating a poorly-adhering coat of block filler with a heavy coating such as an MPI #77 gloss epoxy would only accelerate the failure.

Also, while there were no immediate signs of moisture issues in the walls, the powdery remnants of block filler on the back of the delaminated sheets of epoxy lead the inspector to suspect that moisture had infiltrated at some point. Latex block filler is not water-resistant so it is a poor choice for walls where moisture ingress is likely to occur. If moisture gets trapped between the topcoat and latex block filler, the block filler will turn to mush and then to powder once the moisture dries; it cannot reconstitute itself. Any opportunity for moisture infiltration will readily render a latex block filler the weak link in the coating system.

A Costly Fix

How to repair the failures? This is no easy task: since the facility is an active working lab, all the surface preparation and recoating work has to be done while maintaining a total “clean room” environment; not even a smidge of germ ingress is acceptable. Workers must shower before and after entering and must wear disposable garb to do the work, and all work areas must be sealed off so that no dust or debris escapes into the rest of the lab. The failed coating areas are being removed with hand scrapers; the scraped surfaces are then scrubbed down to remove any vestige of block filler residue. Since the original epoxy coating did its job and was not responsible for the failure, three coats of the same product are being re-applied to the sacked block surfaces.

How to Prevent the Problem

In hindsight, as soon as the architect called for the sacking, the block filler became redundant and should have been eliminated from the coating system; the sacking fulfilled the function of the block filler. A savvy contractor might know to say this, and a qualified paint inspector would have caught the problem. The #77 epoxy topcoat could have been applied directly over the sacked surface, in three coats instead of the two described in MPI system 4.2F. Three coats of epoxy direct to concrete walls is certainly an acceptable system.

And as we discussed in the beginning of this article, should an epoxy block filler (MPI System 4.2G) have been used under the epoxy topcoat, instead of latex block filler? The higher-performance epoxy block filler is certainly preferred because of the regular hydrogen peroxide fumigation exposure. And indeed, had the sacking not been applied, we’d have wanted the epoxy block filler instead of the latex block filler. But once the architect decided to sack the walls, no block filler whatsoever should have been used.

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