MONDAY, DECEMBER 9, 2013
The inspector recently encountered this scenario from a specifer.
“An existing wall has latex paint over an enamel (alkyd) finish. A new latex topcoat was applied that caused the bond between the latex and the alkyd to fail.
"What are the proper repainting procedures? And is the painter responsible for preparing the wall to receive the new paint and primer?”
Now Adhere This!
First, a clarification: the new latex topcoat did not “cause the bond between the existing latex and alkyd to fail.” The existing latex failed long before the new topcoat was applied.
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| Photos: MPI |
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Although the failure wasn't visible until the topcoat was added, it actually occurred long before. |
If an adhesion test had been conducted, the specifier would have found little or no adhesion between the alkyd and the existing latex. And if the existing latex had been subjected to any kind of stress from temperature change, abrasion, impact, or the like, delamination would have already been evident.
We still see this type of failure, even though alkyds aren’t used as much these days. The fact is, even if you sand the heck out of the alkyd before repainting, a conventional latex still won’t adhere.
Clients often ask us: “Can’t I just put another coat of paint on top?”
And the answer is: “No, you can’t.”
If the latex topcoat encounters no stress, you may not know it has failed (it can just lie there and look at you).
But if you apply a fresh coat of latex over it, the lack of adhesion will become readily apparent: As the new film dries, it will exert stress and pull up the latex underneath. Bubbles will appear and, when you open them up, you’ll easily pull up both the new topcoat and the delaminated latex underneath (since these two adhere quite well to each other), revealing the old intact alkyd coating below.
Fixing the Failure
The inspector was once called to a stadium remodeling project that involved extensive repainting.
A very long, large wall that appeared to be in good condition required only a couple of coats for a color change … or so the owner thought. When the work began, however, adhesion issues were immediately apparent.
The inspector determined that while the existing topcoat was a standard latex eggshell, the underlying finish was an eggshell alkyd. Applying a new coat of conventional latex was all the stress needed to delaminate the old latex off the alkyd. The adhesion was so poor that a painter with a knife could easily peel off the latex in sheets.
In these situations, the only solution is to remove the failed latex. Yes, it adds time and expense, but think of it this way: If you had a house with a cracked foundation, would adding another story on top fix the foundation? Or would it make the problem worse? Painting over a finish with no adhesion to the coating underneath is no different.
Do's and Don'ts
The failed latex can be removed mechanically by power sanding or scraping and, on a rounded surface, by sandpaper.
In theory, a chemical stripper could be used. However, selectively stripping off just the failed coatings is difficult, so this is likely to also damage the alkyd finish underneath. In addition, if you use any method other than sanding to remove the failed latex, you must then sand the stripped surface.
The next step is application of a bonding primer. This is an absolute necessity before applying conventional latex paints over an alkyd finish.
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A failed coat of latex over alkyd must be completely removed, usually by power sanding or scraping. |
Back in the day, an alkyd undercoat such as a product approved under MPI #46 was the traditional choice for a bonding primer/tie coat over the alkyd. Nowadays, MPI #69 solvent-based bonding primer—or the more popular low-VOC alternative MPI #17 waterbased bonding primer—are used, followed by two coats of the new latex topcoat.
Making the Grade
The inspector had another project involving an old school that required a seismic upgrade. To keep the school's heritage look, the wood doors and frames were saved and re-installed.
During installation, it was noted that multiple layers of old finish were chipping off easily. It was determined that the original finishes were an alkyd gloss, but school maintenance painters had changed that over the years to a semi-gloss latex. With the stress of removal and handling, the latex layers delaminated off the old alkyd.
The doors and frames had to be scraped and power-sanded and all defects filled with auto-body style fillers. They were then primed with MPI #17 Primer, Bonding, Waterbased.
With all the added time and expense, it would have been far more economical to replace the doors and frames, but the heritage status had to be maintained.
Who’s Responsible?
The second part of the question posed at the beginning of this article asks who is responsible for the added time and expense required to solve the problem. In other words, "Whose fault is it?”
We suggest that the specifier and contractor share responsibility.
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MPI delineates five Degrees of Surface Degradation. Left: DSD-1 (Slightly Deteriorated), with a few scuffs, marks and minor chips. Right: DSD-2 (Moderately Deteriorated), with areas of localized peeling. |
MPI maintains that if you’re going to write a repainting spec, you (or your representative) must first visit the site to evaluate the existing condition of the surface.
Proper practices for evaluating surface condition are described in detail in MPI’s Level 2 Maintenance Repainting training course and MPI’s Repainting Specification Manual. Briefly, however, the process includes:
5 Degrees of Degradation
MPI then offers a five-tier “Degree of Surface Degradation” system. The ratings range from DSD-0 (sound) to DSD-4 (degradation so severe that the underlying surface is damaged and needs to be repaired or replaced).
Both the surface preparation requirements and recommended repaint system are based in part on the DSD rating.
However, MPI also recommends that the painting contractor check the surface condition before beginning work, both to protect his own interests and to assure a successful project.
For repaint jobs, this means evaluating the adhesion of the existing coating, even by simple methods such as applying duct tape or the coin test. Corners, edges, and areas around belly height are the first places to check. Get a scraper and look for loose edges. If you find areas where the existing film readily peels back, there’s a problem.
Caveat Painter
If the contractor still goes ahead and applies a coat of paint to this surface, he has “accepted” it. If problems occur later, he is liable for the remediation or repair. His only recourse at that point is to negotiate with the owner/owner’s rep for reimbursement.
In the worst-case scenario, the painting contractor will end up with a do-over he doesn’t get paid for, if he wants to preserve his reputation and relationship with the customer.
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A cross-hatch adhesion test shows an intact coating. |
It’s far more economical and advantageous for the painter to identify the problem before work starts and bring it to the attention of the owner or owner’s representative. That way, all parties can determine the best course of action and make any changes required to the specification and paint contract.
So specifiers, take note: A good repaint spec is based on visual inspection of the existing surface condition and will include what surface preparation steps or primer may be required before painting. A specification written without inspection that specifies only the finish is fairly begging for problems down the road.
But the painting contractor, considered by most owners to be the “expert” on the project, should do his part by verifying that the surface he’s about to accept is indeed appropriate for the paint specification he’s been given.
Editor's Note: 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; Alkyd; Coating failure; Delamination; Good Technical Practice; Latex; Master Painters Institute (MPI); Paint application; Quality control; Site/field testing; Surface preparation
