Are there professionals in art? When it comes to restoration, the answer is an emphatic yes, as demonstrated by this cautionary story of a well intentioned parishioner who tried to restore a 19th century fresco portrait of Jesus in her local church. Things did not go as she hoped and the image on the right was the result. Professional restoration is an exact science combined with well-trained skills. See below for some sources on how the pros do their work.
Art, biology, and conservation : biodeterioration of works of art / edited by Robert J. Koestler – N8558.5 .A78 2004
Caring for your art / Jill Snyder – N8560 .S66 2001
Practical picture restoration / Peter Oldale – N8560 .O42 1999