Making white (exterior) wall paint stick to mortar rendering

[NY]

Our house is brick built, with mortar rendering over this. The mortar has been painted with white paint - I can't tell whether it's plain paint or whether it has sand in it to give it a texture. That was done by the previous owners before we bought the house two years ago.

Large areas (patches of several square feet) of the paint has suddenly started flaking off the mortar - it seemed to begin at the same time (late last year / early this year) in various parts of the wall. The flakes are several inches square and quite thick as if many coats of paint had been applied. It is the lower parts of the wall (up to about 6 feet from the ground) which have flaked, although on the higher parts of the wall, the paint looks to be cracking and bubbling.

The mortar underneath is sound - that has not started to crack off the bricks.

What is the best way to repaint it? Is it just a case of using a wire brush to get off any loose paint, and then painting with Sandtex or some other outside emulsion paint? Or do I need to first paint the mortar with some sort of primer and then paint that with the white paint?

[nothanks]

We had the same problem on some roughcast render. Wire brush and/or pressure washer, followed by fungicide (which is the bit I suspect the previous owners neglected) before painting.

[nothanks]

Also, don't use a steel wire brush - bits stick in the render and will rust.

[Brian Gaff (Sofa]

Are you sure it was not a fly by night coating firm that did it while it was damp weather and did not use the sealer treatment first. When its done right this sort of finish last 15 years or more, when its not it cracks and come off like large lumps of super sized dandruff. Brian

[Brian Gaff (Sofa]

Another thing, have you done the mallet test on the render? If some areas sound hollow you may need to get the blown render fixed first, and the joints sealed. Brian