Of the companies listed in the 1911 commercial directory of Mitcham, 17 of these were varnish manufacturers.
One of the uses of varnish was as a waterproof coating for maps, as described in this article from 1913.
Waterproof Varnish for Cardboard.
If cardboard is painted with celluloid varnish nice, smooth, washable surface results. This is the varnish with which almost all maps, for land or marine use, are superficially coated, as well as drawings and manuscripts subject to much handling and varying degrees of atmospheric tumidity, even to wetting. The varnishing is effected, either coating the cardboard with the aid of a flat brush and drying in the air, or by dipping. Papers that have been varnished in this way gain after drying 40 to 50 per cent, in strength, and cardboard so treated is perfectly washable, without warping subsequently. Moreover, the coating of celluloid varnish permits the bending of the cardboard in any direction, for it is elastic enough not to break.
Celluloid varnish is made by dissolving celluloid in amyl acetate. The emulsion can be cleaned from waste roll photographic film, and the resultant clean celluloid used. It is simply cut into small bits, placed in the amyl acetate, and allowed to dissolve, shaking occasionally. The usual formula calls for 120 to 150 grains of celluloid to 16oz. of the acetate. But the exact proportion does not matter particularly. Made according to the formula, it is rather too thick, and requires too long to dry. The best way is to let the acetate dissolve about all the celluloid it will take up, and then add nearly enough alcohol to double the bulk of the solution. If too much is added, there will be some of the celluloid thrown down. Made in this way it is much cheaper, dries much quicker, and flows better, being thinner.
—“ Camera Craft.”
Source: Sheffield Weekly Telegraph – Saturday 15 February 1913 from the British Newspaper Archive (subscription required)