External Wall Surface

[Matheous-2]

We have a problem with an area beside our front door which is 'raised' compared to the roughcast on the rest of the building. The surface is powdery light grey in colour and shows traces of rust! It does not accept external type paints which bubble after a few months.....Any suggestions would be appreciated. we would just like it to have the correct type of paint......

[AlBags]

Well, I don't know how big the area is ...
Can't it be knocked off and re-rendered with mortar.
Than can refinish/paint properly.