I have written before in Render unto God … about the fact that many medieval English parish churches, as well as castles and town walls, were originally rendered and painted to seal the walls against damp ingress. With ashlar built churches this was not necessary, but in areas lacking good building stone it was a practical necessity. Where ashlar was used in architectural details such as quoins and other details that masonry was left exposed to provide contrast.
This is a topic which is not discussed much in books on parish church architecture, and overly enthusiastic nineteenth century restores often stripped surviving rendering off medieval churches along with the internal plaster. The exterior may now look pleasantly rural and traditional, but it is not what the builders intended.
I came by chance upon a video about this subject, and looks at several examples. The practical need to protect the fabric against the weather is shown, and some of the comments make interesting points. It can be accessed at Thundridge Old Church
The history of the church at Thundridge is introduced on Wikipedia in articles which can be seen at /Thundridge and in another about a hamlet in the parish at Cold_Christmas
The website of a voluntary trust which seeks to preserve the tower can be accessed at Protect and Share Thundridge Bury History
Amongst its illustrations is a charming view of the church before its demolition in 1853. Its loss appears very regrettable indeed.
Examples of surviving medieval rendering on castles can be seen at Conwy and on the shell keep of Totnes. These were castles that were meant to be seen with their gleaming white walls, not structures that blended into the landscape. A later twelfth century reference to a newly built castle within the Angevin lands in France lays stress on its white walls.