Ask the Expert Question-and-Answer Archive

by Ron Joseph

December, 2004

New Paint Blistering

Q. We are in the process of having our house repainted. The painters performing the work have been doing typical prep work in terms of sanding, priming, and caulking spots that needed attention. We live in Austin, TX. Certain parts of the trim on the house were spray painted several weeks ago when it was warm and seem to be performing fine. Other areas were painted last week when temperatures ranged from 40's at night to 50's and 60's during the day. To my knowleged, there had been no rain for at least a week, so the house surfaces appear to have been thoroughly dry before they were painted. Several days after this painting has been performed, we have been having steady rain. Areas on the side of our unheated detached garage and columns below a second story porch have been thoroughly dampened by the rain. We are now seeing blistering of these surfaces with moisture appearing to be trapped in the blisters. Is this a paint failure or more likely an application problem? How should we remedy this problem?

I'd like to express my thanks in advance for any feedback you would be willing to provide.

It sound as if it may be a combination of a few things that is causing the paint to blister. Moisture is migrating up from behind the paint and causing the blisters to form. If all of the exposed edges have not been properly sealed, the moisture will seep behind the paint, then wick through the substrate to the surface and form the blister. If there is an insufficient amount of paint to properly protect the substrate, the moisture can migrate through the paint film and into the substrate and wick up through and form a blister.

If you consider what it is that we want the paint to do, we are expecting it to create a water or moisture barrier to protect the underlying substrate. If there is any voids in the film, poor surface prep, or if there is an insufficient application of coating to provide the desired protection, then we should expect an insufficient level of protection.

The application of paint in these areas should be revisited. First allow the substrate to completely and sufficiently dry, any residual moisture will find its way out to the surface behind the new paint and the blistering will reoccur. Take extra precautions to insure that any and all exposed edges are sealed, either with caulking material or with plenty of paint. Remove any and all loose paint, feather out any sharp edges with sand paper, and clean the surface by removing all dust, dirt and debris.

Finally, only apply the paint when the ambient conditions are favorable, such as warm, dry weather, in order for the paint to properly dry and form a good protective film. If the conditions are not favorable to the application and drying of latex or waterbased paints, (such as high humidity, rain or heavy moisture or cold temperatures) the solvent portion of the paint will evaporate first, leaving the water in the formulation to come off last. This too will cause the paint film to be inferior.

Regards,

Jim Burke