Q. We manufacture molding for the RV industry using specialty laminated wood from China that has been extruded with “gesso” that we then coat with water-based paints. Next, we wrap the molding with various papers and veneers using PUR (polyurethane reactive) glue on a glue machine line. Lately, we have been experiencing some delamination of these materials down to the gesso layer. As we looked closer, at we noticed that the paint we are using is sticking to the PUR glue bond on the backside of the delaminated veneer/papers. In your experience, do you know what could be the issue?—B.L.
A. This is not as uncommon a problem as you would think. First, you need to understand what “gesso” is. Gesso, by definition, means chalk, and it has been used for hundreds of years to size or level wood and metal for processes such as paint, gold leaf, etc. Many of today’s gessos are made with materials ranging from calcium carbonate (CaCO3) to kaolinite (Al2Si2O5(OH)4). These powdered materials are typically mixed with aqueous or oil bases, then applied with an extrusion die machine process and allowed to thoroughly air-dry.
I will highlight a few areas that may be of help in finding a solution to your situation. As with any laminating process, your laminating bond is only as good as your weakest link. First, do you sand the parts in question? If so, are you adequately removing any sanding dust and debris? This may be an issue because, more than likely, the parts from China may be using gesso made with kaolin (china clay), which contains silicones found naturally in kaolinite. If this dust/debris is not removed adequately, it can create adhesion problems with the water-based paint.
Second, gesso aside, you also mention what appears to be paint on the PUR glue bond, so I would like to propose a few simple tests you could conduct to validate the paint’s adhesion properties. After you have sanded the gesso and successfully removed the dust/debris, simply spray a few pieces of the laminated/gesso substrate with the paint in question and allow it to dry completely. For Test 1, use a razor blade to make a series of lines in the dried, painted surface 1/16-inch apart in both directions and approximately 1-inch square. Next, take a clear piece of 2-inch packing tape and place it over the top of this grid, rubbing it into place. Finally, grab a corner of the tape and quickly pull it off. Test 2 is similar to the first test, but apply the tape directly onto the surface without any razor cuts, and then pull it off as in the first test.
With either test, observe what is on the tape. Do you see little paint squares or gesso in Test 1, or whole sheets of paint in Test 2? If the gesso stayed in place on the laminate but the paint came off with either test method, this is a good sign that there may be a paint adhesion issue over this particular gesso formulation.
Lastly, to further validate whether the paint could be the culprit, you can conduct a third test by applying the various laminates directly to the gesso surface without any paint. If the laminate bond is strong and there does not appear to be any issue with adhesion, maybe you could eliminate the paint step altogether, unless there is a specific reason it is needed. However, if a water-based paint is required, you may want to consider contacting your paint supplier and experimenting with other types of paints that would promote better adhesion to this particular gesso.