Varnish is an important component of traditional oil paintings on canvas; if its surface gets damaged, its protective role no longer serves its purpose and the aesthetic value of the work of art is reduced. It has been proved that all varnishes based on natural resins as well as synthetic resins deteriorate faster if stored under inappropriate storage conditions. The changes/damage are the result of chemical and/or physical environmental impacts on the materials. Some resins are more sensitive and thus deteriorate faster than others, which is why it is important which finishing varnish is used on the work of art that is to be returned back to the unstable environment after the conservation and restauration works. For this reason, this master’s thesis deals with the conservation and restauration treatment of deteriorated finishing varnishes as well as the changes/damage, which are terminologically classified. What is more, the significance of the preventive conservation as ensuring an appropriate environment for storing and exhibition of paintings on canvas is emphasised. The thesis is based on the painting of St. Judoc in the Church of St. Lucia (in Begunje na Gorenjskem) that was severely damaged due to inappropriate storing. In analysing the painting, the question on the appropriate protection after the conservation and restauration works was raised.