| Yellowing of white or off-white interior latex paints after they have been applied over new drywall is not a new problem. However, we seem to be hearing and seeing more examples of this problem. The affect is that all tape joints will appear whiter than the drywall, causing a banding effect. There are differing opinions as to what causes the yellowing to occur. It should be noted that yellowing is a generic term and that other shades of discoloration can occur.
Some believe that a new kind of sizing compound that is used by the drywall manufacturers to treat the surface of the cardboard facings could be a cause. This new sizing contains a very high alkaline content that can react with latex paint, causing discoloration. While this could be a contributor to the yellowing problem, there are some drywall manufacturers that do not use a high alkaline sizing on their product, and yet yellowing still occurs in some instances with their product.
Another theory is that the recycled paper being used today for the facing cardboard is manufactured from a variety of materials instead of the high grade of recycled paper that was used in the past. Today's recycled paper can contain wood pulp and other low grades of recycled materials including wood by-products. Much like the exterior tannin staining on wood substrates, these wood by-products can contain tannic acid that is known to be a water-soluble staining agent that can discolor latex paint.
Moisture seems to be at the center of the problem. The atmosphere during the paint application and drying process can affect the degree of discoloration. The problem seems to occur more frequently if a contractor is painting a house in a cooler climate, and the house is closed up tight at night. The latex coating stays wet for an extended amount of time. There is high humidity from the water in the paint, and this soaking on the drywall surface will allow those tannin stains to bleed out of the drywall.
To help prevent this situation, a lot of air movement and heat needs to be created so the coating will dry faster and is not allowed to soak on the substrate. Latex paints need to dry under consistent atmospheric conditions, to help negate the effects of discoloration.
Joint compound and drywall texture must be allowed to fully dry before applying a coating. It is recommended that 24 hours drying time be allowed so the compound can fully dry. On the surface it may be dry, but underneath, moisture could still exist. "Gray Lining" is the term used to describe the discoloration that occurs. To wait might put pressure on the drywall installer and the painter, but it's better than having to go back and redo the project.
At this time we have no easy solutions to prevent yellowing from occurring. There are steps a painting contractor can take to help reduce the likelihood of this problem.
1. Don't thin your paint. You need all the covering capacity possible to contain leftover pigments that may bleed through.
2. Our best recommendation is to use DU-1504 Interior Latex P.V.A. Primer/Sealer. Many times a single coat of latex is applied followed by the finish coat. This single coat of latex will not cover like a primer. Some drywall manufacturers recommend the use of an oil based primer to control drywall staining. It should be noted that using an oil based primer on new drywall may raise the nap on the surface. A light sanding is recommended between coats, to control this. If an oil based primer is needed, we recommend CU-1401 Interior Alkyd Enamel Undercoat.
3. Always allow ample drying time between coats and before your first coat over the joint compound. Pay attention to the environmental conditions both outside and inside to determine if a situation exists that might lead to yellowing.
4. When in doubt, contact your Diamond Vogel sales representative.
For more information regarding drywall finishing, consult the Diamond Vogel product guide under "Surface Prep & Application" or contact your Diamond Vogel representative.
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