How do I repair cement rendering?

[Handyman]

Problem: Multiple garage units rendered with sand/cement screed and painted with masonry paint. In some areas, the paint has flaked and the salty atmosphere has got in behind he paint and eroded the top surface of the rendering, sometimes to a depth of up to 10mm, and leaving the surface very rough.
Question: What materials are available to rebuild this surface back to original level, before I repaint? Re-rendering with sand & cement is not practical, as thickness is variable, and insufficient to guarantee a good bond. I'm looking for something like an external plaster which I could use to skim the eroded areas. Someone has suggested one of this viscous, high-build, vinyl coatings (like thick emulsion) which (according to the can) are suitable for external use, but I'm concerned that this will just smooth-out the imperfections rather than rebuild the surface.

Final surface finish needs to be only as good as a normal sand/cement render, and not as smooth as what the vinyl stuff might give.

Any advice much appreciated.

 

[Building Contractor]

I think you will have to look the the areas that are really bad and chop them back just enough to get a key to be able to render them again with a fine washed render sand, not forgetting to seal the wall first. Then add a little render sand to a good external paint to blend it all in.