To make the conservator's job interesting, there is no one agent that will clean all paintings of all types of dirt or varnish. Paintings are cleaned with a range of different agents that are tailored to suit the type of painting and substance (dirt or varnish) that is to be removed from the surface. Tests are carried out to find out which ones will most effectively remove the dirt or varnish and whether they are safe for the painting. These tests are repeated throughout the treatment to take into account different areas of the painting, the different painting techniques used by the artist, different colours, and different levels of dirt.

Under no circumstances should it be attempted by the non-expert. Even the most straightforward looking painting can go on to reveal a host of problems (for example flaking paint). Too many things can go wrong, and can cause irreparable and expensive damage. Tests are exhaustive, and any decisions are backed up with observations and with experience.

Dirt can be removed with water based agents. One of the agents that can be used is saliva - it is believed that the enzymes help to remove dirt! As water can be harmful to paintings, only tiny amounts are used. Discoloured varnish is removed with a range of organic solvents, either mixed or used individually to suit the type of varnish.