Fragments of the curtain wall with what may be whitewash are known, as are collapsed sections of what appear to be rendering. Examples of rendered walls are known from the Roman world, sometimes scribed and painted with red lines to look like ashlar masonry, sometimes just painted.
However, the evidence for Hadrian’s Wall is by no means clear-cut, and so-called examples of whitewash could easily be due to leaching of lime from mortar whilst the rendering could be a type of pointing known as ribbon pointing. Examples of the possible types of external finish of the curtain wall are displayed on the west end of the reconstructed wall at Wallsend: ribbon pointed, rendered and scribed, rendered and painted, and whitewashed.
Further reading: Breeze 2006