This report, Conserving Historic Signs, was written in 2002 and published by the Heritage Office, Department of Planning, State of New South Wales in 2006. I was alerted to it by Jos Peeters and it is a detailed and fascinating account of some of the considerations when conserving ghostsigns. The work on the HAT archive seems to fall under what is listed as the ‘minimal’ form of conservation by using photos to record what currently exists.
I’m going to follow up some of the references so any help there much appreciated. Also, any other related articles or other resources gratefully received.