THURSDAY, JULY 24, 2014
Second of Two Parts
The lab was brand new; the sacking, superb; and the coatings, exactly as specified.
Nevertheless, a year later, the paint system fell completely apart. Why?
Because no good deed goes unpunished? In this case, it sure felt that way.
To recap: This case involves the coating of interior concrete block in a new "clean room" laboratory. To address the porous appearance and texture of the light-density block, the surface was skillfully sacked. The specified system (latex block filler, followed by two coats of epoxy) was then applied.
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| Photos: MPI |
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They thought they did everything right. After a year, the walls told otherwise. |
A year later, the epoxy topcoat was peeling off in sheets.
Here's 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 holes. Its adhesion is based on its application into a rough and porous surface. If it simply lies on top, adhesion will be poor.
We’ve seen problems like this before, when concrete block is part of (or connected to) a tilt-up or other type of concrete wall. The contractor takes the filler he was applying to the block and also applies it to the wall.
Too late, he learns that block filler will not stick to concrete. After topcoating, there’s no adhesion between the block filler and the wall. The block filler will shear off, taking the topcoat with it.
Sacking + Filler = Failure
Knowing this, the paint inspector in the lab observed the sheets of delaminated coating and deduced thus: Once the block surface was filled with a good coat of sacking, the need for block filler was eliminated—and so was any chance that the filler would adhere to the surface.
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The layer of block filler can be found on the back of the sheet of delaminated epoxy. |
Furthermore, even though everything looked fine after the initial coating application, topcoating a poorly adhering layer of block filler with a heavy coating such as an MPI #77 gloss epoxy would only accelerate the failure.
Wily Water
Also, while there were no immediate signs of moisture in the walls, the powdery remnants of block filler on the back of the delaminated epoxy sheets led 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. If moisture gets trapped between the topcoat and latex block filler, the 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. Because the facility is an active working lab, all of the surface preparation and recoating work has to be done while maintaining a total “clean room” environment. Not even a smidgen of germ ingress is acceptable.
Workers must shower before and after entering and must wear disposable garb while working. All work areas must be sealed off to prevent the escape of dust or debris into the rest of the lab.
The failed coatings are being removed with hand scrapers, and the scraped surfaces then scrubbed to remove any trace of block filler residue.
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With shearing failure, the epoxy topcoat readily delaminated from the block filler beneath it.   |
Because the original epoxy coating did its job and was not responsible for the failure, three coats of the same product were re-applied to the sacked block.
Preventing the Problem
As soon as the architect called for the sacking, the block filler became redundant and should have been eliminated from the coating system. A savvy contractor might know to say this and, in hindsight, 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 yesterday's article, should an epoxy block filler (MPI System 4.2G), rather than latex, have been used under the epoxy topcoat?
The higher-performance epoxy is certainly preferred because of the regular hydrogen peroxide fumigation exposure. Indeed, had the sacking not been applied, we’d have wanted the epoxy instead of the latex filler.
But once the architect decided to sack the walls, no block filler whatsoever should have been used.
About the Author: 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: Coating failure; Coating inspection; Concrete masonry units (CMU); Failure analysis; Good Technical Practice; Master Painters Institute (MPI); Specifiers
