Maintenance repaints are easy, he told me. Just wash the old topcoat and apply another coat of the same product and the new coating should be good for a few more years. Such was the belief of the engineer who wrote the specification for recoating the exterior of four large tanks.
Unfortunately, during 10 years in service, the zinc primer had reacted with the topcoat and weakened its bond to the primer, and the coating manufacturer had updated the formulation of the new topcoat material. The plasticizer in the new topcoat softened the old topcoat and further weakened its bond to the primer. The shrinkage stress that developed as the new topcoat cured caused the bond between the old topcoat and the primer to break. After six months, both topcoats were hanging down the sides of the tanks like sheets of wet wallpaper. It was an expensive way to strip a tank for recoating.
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The above case might seem extreme, but failures after maintenance recoating occur more frequently than coating failures on new construction, especially when applying new material over old coatings. If multiple coats have built up on old structures, often there are planes of weakness between some of the coats and these are prone to fail under the stress imposed by a freshly applied material.
Test and evaluate
The best way to avoid coating failures is to plan at least one year ahead. Having decided on the best options for repainting, try the proposed procedures and materials by applying test patches. The patches should be at least one square foot. If different types of service environment are involved, use one patch for each environment. For exterior coatings subject to winter freeze, it’s usually sufficient to apply the patches in the fall. Anything that can go wrong usually will have gone wrong by the spring. In climate controlled environments, allow at least a year for failures to show up.
Evaluate the adhesion of the existing coating before applying the test patches. Test the adhesion of the combined coatings about a week after applying the patches and at the end of the test. Evaluate adhesion by using knife cuts and adhesive tape or just by using a sharp knife. The two methods are described in ASTM D 3359-09 and ASTM D 6677-07, respectively. The first test requires cutting either a 45° cross-cut or a cross-hatch pattern into the coating, applying adhesive tape over the cut and peeling the tape off to see how much coating it removes. The second method requires just a 45° cross followed by probing with the point of a knife.
Keep records to plan recoats
You need to know the identity of the existing coating system and its overall condition as well as the substrate type and condition to select an appropriate maintenance recoating procedure. Hopefully you’ll have logs and inspection reports from previous coating operations. You also need to know the characteristics and limitations of the new coatings that are possible candidates for the project.
If you’re fortunate, you’ll find records showing the identity of the existing coating system, but many times this won’t be the case. A school district had the corridors in its schools painted with graffiti-resistant paint, then lost the record. No doubt the janitors knew that nothing would stick to the walls, but not the architect who specified a fresh coat of acrylic latex as part of a redecoration project. Needless to say, it wasn’t a success.
Some coatings are just incompatible with one another. For example, alkyd paints (oil-based) are easily softened by the strong solvents in many two-component coatings such as epoxies and polyurethanes. If these are applied over old alkyd coatings, they might act like paint strippers and peel the old paint off the substrate. Also, many water-based latex paints might not establish good adhesion to old oil-based coatings. Of course, these problems will show up when test patches are applied, but time will be lost.
Knowing the type of existing paint is an important part of choosing a product that can be applied over it successfully. ASTM D 5043-04 describes a procedure for field identification of coatings, but it requires access to a variety of chemicals. Chips of the coating can be identified by a suitably experienced laboratory using infrared spectroscopy, and it might be possible to get the candidate coating supplier’s laboratory to do so. But keeping good records of coating work avoids the problem.
Factory finishes require finesse
An especially difficult problem is maintaining factory-coated siding and roof decking. A galvanized steel substrate has a thin, baked on layer of specialized coating. These coatings include polyesters, silicones and the toughest recoat problem of all, Kynar.