When paint is blistering and peeling, laboratory analysis is often required to determine the cause. But what types of samples are needed, and how should the samples be collected? These are important questions, as the greatest sources of error faced by laboratories when analyzing coating failures occur before the box of samples is even opened. Problems arise related to the types of samples that have been collected and the care that was exercised during collection.
Oftentimes, the laboratory receives a bag of paint chips that were picked up from the ground beneath an area of peeling paint located 20 feet overhead. There’s no proof that the chips came from the area in question. Further, the lab has no way of knowing whether the chips have been contaminated while lying on the ground, if rainwater washed away critical evidence or if the cause of the peeling is still 20 feet overhead.
Another flaw comes from limiting the samples to failing areas only, without also providing samples for comparison from zones where the coating is performing properly.
This article addresses some of the common problems associated with sampling.
SAMPLES OF FAILING PAINT
When removing samples of paint, avoid touching them with your bare hands, and when possible, collect a minimum of four square inches of coating at each sample location.
To minimize the potential for external contamination when taking samples of peeling paint, look for defective areas that have not yet become completely detached. For example, if a paint system is separating between coats, look around the perimeter of the peeling area for coating that is loose but still in place, so that the weak interface is more protected from the elements (Figure 1). If the peeling paint stops at a tight edge, you have no choice but to collect samples that have already been exposed; however, avoid including chips that show visible evidence of external contamination, such as staining from water runoff. In any case, if possible, do not collect samples from the ground.
In peeling or blistering areas, collect two samples at each test site. Tape or hold a Ziplock-style plastic bag under the sample area, remove the detached coating, drop it in the bag and seal it. Tape or hold a second bag beneath the same test area, and collect samples of the coating that remains on the surface. In this case, try to cut under the coating to the substrate to provide as large a surface for analysis as possible. The cause of the failure may be residing on the surface of the underlying coat, so the larger the undisturbed surface, the better (Figure 2). If chips cannot be removed, scrape the coating with a knife. If the failure occurs down to the substrate, scrape any material from the surface of the substrate into the bag.
If the failure involves blistering, samples of the blister liquid often contain a great deal of evidence regarding the failure. Find an intact blister (Figure 3) and wash the coating at the top of the blister with deionized water to remove any surface contamination. Use a clean syringe to puncture the top of the blister and extract the liquid. Place the liquid in a small, clean glass vial. Laboratories can supply septum vials for this purpose. If glass vials are not available, place the entire syringe in a plastic bag, completely seal it, and place it in another bag and seal it as well. Once the blister liquid is removed, collect samples of the disbonded coating and the underlying coating/substrate as described above.
Collect samples from representative locations across the surface and, if different types of failure are present (for example, blistering and peeling), be certain to collect samples from all types. The cause(s) of the failures may be different.
SAMPLES OF INTACT PAINT
Samples of intact paint should be removed to provide examples of a properly performing coating. Again, the sample locations need to be representative of the structure. Try to work the knife blade under the coating to remove as large a sample as possible.
Another way to collect a sample is to apply tape (such as duct tape) to the surface. Make parallel cuts through the tape and coating to the substrate. The spacing between the cuts will depend on the thickness and adhesion of the coating, but start with approximately ¼-inch spacing. Once a series of cuts is made (say eight), firmly attach another piece of tape over the area. Make another series of parallel cuts through the tape and coating perpendicular to the first set to create a grid, then apply another piece of tape to hold it all together. Then slowly work the tape with the coating attached, from the surface. The result will be a grid of ¼-inch squares of coating attached to the tape.
The laboratory will now have a large surface of the underside of the coating to examine, and it can also carefully work down through the layers to examine each coat as necessary. Note that if an analysis of the top surface of the coating is required, this technique should not be used because the adhesive from the tape will have contaminated the surface.
SAMPLES OF EACH COAT
If it is anticipated that an analysis of each coat will be required, the laboratory will be limited if they only receive scrapings of the total system. Depending on the generic type of coatings involved, it will be difficult for the laboratory to isolate a single coat from small shavings that include all coats. It is often easier for the separation of the coats to occur in the field, provided there is a contrasting color between coats.
This can be done by removing the blade of a box cutter and holding the sharp edge of the blade flat against the surface. Hold the blade at an angle and carefully scrape it across the surface to shave or powder off the finish, being careful not to gouge into the underlying coats, and collect the shavings into a sample bag as described above.
For example, if the finish coat is white and the intermediate coat is beige, scrape the white finish until beige is exposed. Move an inch away and repeat the process. Once the sample of the finish is collected, seal the bag. Scrape and discard as much of the remaining white finish in the sample area as practical until the surface is predominately beige. Open a new sample bag and scrape only the beige into it.
Repeat the process for each coat. Even though there will be some intermixing of the coats in the sample, the laboratory can examine the fine powder under the microscope and remove the undesirable particles before conducting the analysis.
THE BEST SAMPLES OF ALL
Since you cannot bring sophisticated laboratory equipment to the site, try to bring the structure to the laboratory. If the issues are related to coatings on concrete masonry units, see if it is possible to remove a portion of a block face with the coating attached. A hammer and chisel can also be used to remove a chuck of concrete with the coating attached, or to remove the corner of a block (Figure 4).
If applied to metal, it may be possible to remove a component (a bracket, for example) or cut a small piece from the end of the component. In this way, the laboratory technician can carefully examine the samples under controlled conditions while viewing under a microscope, and selectively remove layers — a task that, as indicated above, is often impossible to perform when dealing with small chips.
The technician can also examine a freshly disbonded surface without fear of contamination. When failures extend to the substrate, examinations of the substrate itself can be much more complete. There are countless reasons why samples of paint with the substrate attached are ideal for laboratory failure analysis.
A unique sample ID number, the date and time the sample is collected, sample location, a brief description of the sample, and the name and signature of the person collecting the sample should be documented on a Chain-of-Custody form. Laboratories can provide Chain-of-Custody forms, and they can also be found online. The sample ID, sample location and date should also be recorded on the sample bags themselves.
Photographs of the sample sites should be taken (Figure 5). Ideally, a photograph of the general area should be taken to put the sample location — and what it represents — into perspective. Close-up photos of the sample site before removal, the sample site after removal and a photo of the bagged samples at each site should be taken.
A ruler or some other means of providing scale should be included in the photos, together with some descriptive information, such as the sample ID number, date, location and perhaps a brief description. This information can be written on the surface next to the sample, or on a piece of tape that is removed after the photographs are taken.
Great care needs to be exercised when collecting the samples, as well as in the selection of sample sites to make sure they are representative of the issues that are present. Collecting large, undisturbed, uncontaminated samples goes a long way toward helping to determine the cause(s) of problems. Samples of intact, properly performing coating should also be taken to serve as a basis of comparison.