Blistering

Possible Cause

Blistering can be caused by:

• Painting a warm surface in direct sunlight.

• Applying oil-based or alkyd paint over a damp or wet surface.

• Moisture escaping through the exterior walls (this is less likely with latex paint than oil-based or alkyd paints).

• Exposure of latex paint film to dew, high humidity or rain shortly after paint has dried, especially if there was inadequate surface preparation.

Solution

Old painted surfaces will require adequate surface preparation. The first step is to carry out a series of “adhesion tests” to clearly determine the quality of the existing paint work. Simply cut a small X through the existing paint with a sharp blade, press cellulose tape firmly across the cut and then rip off the tape. If the paint comes off, it is unsound and should be removed.

If blisters go down to the substrate, first try to remove the source of moisture. Remove blisters by scraping, then sanding the surface.

Prime any bare timber with a high quality water-based primer, and repaint with a high quality water-based exterior paint. Dulux recommends Dulux 1® Step Acrylic Primer Sealer Undercoat and Dulux Weathershield®. For more information, please consult our detailed Technical Advice note on the topic.

Prevention

Paint application substrate must return well under 10% moisture content; any moisture ingress must be isolated. Trapped moisture within substrate will compromise new paint system performance and result in blisters and eventual paint system delamination.