Burning

Cause
Stucco and mortar joints contain hydrated or "hot" lime, a dry compound added to improve the workability of these materials. Different mortars are compounded with different amounts of “hot” lime, depending on the end use of the mortar. This problem is then made worse by more “hot” lime being added at the job site.

The alkalinity of mortar, cement mixes and concrete is expressed numerically by pH readings. The pH of fresh cementitious materials is 13 to 14. It takes approximately 30 days for fresh cement materials to drop to an approximate pH of 9. A pH of 9 is less alkaline than a pH of 13, and a coating has a greater assurance of success when applied to a less alkaline surface.

Once these cement walls are cured, the introduction of moisture into these walls will reactivate the high pH alkalinity, resulting in efflorescence, blistering and peeling of the paint film.
 
Solution

When damp areas are found on old surfaces, the source of moisture intrusion must be repaired. The surface should then be prepared by removing chalk, efflorescence and damaged paint film, and must be thoroughly dry before applying new paint.

Both new and aged unpainted walls must be clean and free of moisture and efflorescence. (See Efflorescence for description and resolution.)

Paint products must be chosen with care. The alkalinity of "hot" surfaces will destroy an alkyd or oil product within 6 months. A vinyl acrylic coating will perform slightly better; however, it will ultimately fail. A 1000/a acrylic coating will give the best service performance.

Flat latex 100 percent acrylic coatings will perform best because of their ability to allow vapor transmission through the dry paint film. Non-permeable films (e.g., gloss paints) may blister when applied over the same damp condition.

Note: Composition board or hardboard must be sanded carefully to avoid damaging the surface. Use extra precautions when removing paint film from these surfaces.


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