Our house is brick, coated with mortar rendering that has been painted with Sandtex or something similar: sand in white external emulsion paint. The house was built in 1993, so the mortar and maybe the paint were probably done at that time.
On some parts of the wall the paint is bubbling and flaking away from the mortar. Flakes are anything from a couple of millimetres to several centimetres in size. The bare mortar underneath looks and feels to be sound: it's not damp and it's not bubbling away from the bricks. There are a few large cracks in the mortar, but the location of these seems to bear no correlation with the areas that are flaking.
What is the best treatment? Is it sufficient to fill any cracks in the mortar, and use a paint scraper to remove all loose paint (leaving any which is firmly attached) and repaint with Sandtex? Does the mortar need to be primed with anything before being painted?
How good is mortar at sticking to white Sandtex? When we had some building work done, a new door was inserted and the lintel over it was covered with mortar and painted. But there are a number of short grooves and indentations (*) in the mortar (it wasn't really finished off very well!). The builders painted over those as part of painting all the area of their work. Will new mortar (or external-grade filler) adhere to paint, allowing the grooves to be filled and then repainted? If not, what's the best way of removing the paint to allow mortar-to-mortar contact?
The builders seem to have disappeared off the face of the earth. They had finished the work that we asked them to do, but there was other work that they were still investigating and quoting on (**). We didn't chase them because Covid was just starting, but since then all phone messages and emails have gone unanswered, although their website is still being updated. We still have their industrial vacuum cleaner that they left - you'd think they'd want that back!
(*) Probably a few cm long and about 2 mm deep. They look like score marks caused by small stones or something digging into the mortar as it was being smoothed.