A method to remove scratches.
Scratches on the base side of the film can be either polished or matted. Films can be polished by either the classical method or by the coating process. Matting is rarely carried out, as afterwards it is scarcely possible to print the film optically (because of loss of light and strong reproduction of the rough surface of the base when printing by a directed beam of light). In case of severe base scratches it is possible to polish the base after matting it.
Classical method of polishing (sometimes referred to as coating)
The base side of the film is drawn under tension over a polished glass roller which takes a mixture of, for example, acetone, ether and methanol from a supply trough. The polishing process is initiated by the softening of the base side in the solvent mixture. When the film is pressed very tightly against the polished glass roller the scratches are closed up. The film takes on the smooth surface structure of the roller.
Polishing by the Coating process
Here the film base is treated on the picture width only, with the aid of a non-corroding metal roller which dips into a solvent. There is no direct contact between film and roller. The film’s base side is moved past the roller at a distance of about 0.5mm. The roller dips into a trough containing solvents, which are carried along by adhesion to make contact with the film. To ensure uniform coverage the roller moves at twice the speed of the film. In the drying cabinet the solvent is able to work on the film base and dry off (evaporate). The solvent does not operate as in the polishing process where the scratches are pressed together, but instead the refracting sharp edges of the scratches are rounded, so that refraction no longer takes place. The film is therefore practically unscratched for projection or printing purposes. Beware that this method will increase the incidence of buckle and weave.
There are also devices which can apply protective lacquers by the coating method to increase the film’s abrasion resistance.
FIAF, 1986, Preservation and Restoration of Moving Images and Sound, Federation Internationale des Archives du Film, FIAF, Brussels, Belgium