Roger in the US wrote:I am doing some painting of an area where the original paint "lifted and bubbled" from the concrete. I am guessing that the concrete was not mixed well when originally was poured. I removed the old paint and scraped and sanded and repainted but the same thing happened again.
If this happened here in Peru, especially in the first floor, your problem most probably is "Salitre
Behind the lifted and bubbled painting you should find a white powdery bloom defect.
If this is the case, sanding and repainting will not
Usually, in such case, one has to remove the entire plaster/render (called "tarajeo") arround the affected area and apply a new plaster with a very rich plaster mixture (cemento + fine sand 1: 4 mixed with impermeabilizante such as SIKA or CHEMA!).
If the wall is humid, even this will not completely solve the problem. However, it may work for the next 2 or 3 years.
This is due to the fact the sand here in Peru usually contains salpeter (salitre) and the majority of the Peruvian houses are built directly on top of the soil without any capillary breaking layer.