Repaint Specs and the Perils of Latex Over Alkyd

Based on the experience of Paint Quality Assurance Inspector Dave Lick and MPI’s new Maintenance Repainting on-line training course. For information on the course and how to become an MPI-Certified Maintenance Repainting Coating Specialist, click here.

The inspector recently encountered this question 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?”

No, You Can’t Just Paint Over That

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. 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, etc. delamination failure would have already been evident.

We still see this type of failure even though alkyds aren’t used as much these days. Fact is, even if you sand the heck out of the alkyd prior to 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’s failed – it can just lay 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 as well as the delaminated latex coat underneath (since these two adhere quite well to eachother!), revealing the old intact alkyd coating below.

Protocols for 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 commenced, adhesion issues were immediately apparent, and the inspector determined that while the existing topcoat to be painted over was a standard latex eggshell, the underlying finish was an eggshell alkyd. Applying the 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.

The failed latex paints 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 these are likely to also damage the alkyd finish underneath. 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. 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.

The inspector had another project involving an old school that required a seismic upgrade. To keep the heritage look of the school, 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 over the years, the school maintenance painters had changed over to a semi-gloss latex. When the doors encountered the stress of removal and handling, the latex layers delaminated off the old underlying 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 with new ones, but for this project, 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 -- essentially, “whose fault is it?”

We suggest the responsibility is shared between specifier and contractor.

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 as well as MPI’s Repainting Specification Manual; in brief, they include checking the thickness and adhesion of the existing coating; researching which paint products were used previously; assessing what defects the existing coatings exhibit; and determining how degraded the finish is. MPI then offers a 5-tier “Degree of Surface Degradation” system ranging from DSD-0 (sound surface) to DSD-4 (degradation so severe that the underlying surface is damaged and needs to be repaired or replaced), and both the surface preparation requirements and recommended repaint system are based on what DSD rating the surface is given.

However, MPI also advocates that the painting contractor should check the surface condition before beginning work, both to protect his own interests as well as 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 are 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.

If the contractor goes ahead and applies a coat of paint to this surface, he has “accepted” it and if problems subsequently occur, he is liable for the remediation or repair work. His recourse at that point is to negotiate with the owner/owner’s rep for reimbursement and 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.
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, so all parties can determine the best course of action and any changes required to the specification and paint contract.

So specifiers, take note: a good repaint spec is written based on visual inspection of the existing surface condition, and will include what surface preparation steps or primer may be required prior to paint application. A repaint 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.

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