Interesting post that got me to looking and finding this:
DAVAC Gumming A new gum from 1963 aided production of Gold Coins In issueDS31, your compiler used the term 'wateractivated' to describe the gumming used on the Tonga Gold Coins stamps. This is the term that is invariably used by stamp printers to signify stamps that require either saliva or a wet sponge to activate the gum on the reverse. This term was unfamiliar to more than one reader, so if you were one of them, it is now hopefully clear to you.
Space on the front page prevented full details of what was then a new gum type from being described, for while the Tongan issue was wateractivated for sure, it actually comprised a new invisible gum called DAVAC. The producers, Samuel Jones, wrote at the time.....
DAVAC is a clear, matt adhesive. Made and pioneered by Samuel Jones for Walsall's Tonga coins of 1963. A suspension of discrete particles of adhesive in a non-aqueous solution of a resin binder, which is then coated on a paper and the solvent dried off. This leaves a paper, on the surface of which lie the small individual particles of the water soluble adhesive anchored to the body paper by a small proportion of resin. On moistening, the binder permits the water to permeate through to the adhesive particles, so giving a tacky gummed film indistinguishable from normal gumming.
The advantage of DAVAC is that it does not cause the paper to curl under extreme climatic conditions. It also enables specialised papers and foils to be used, which could not normally be processed by mechanical non-curling processes.
The "mechanical non-curling processes" that are referred to above are the 'gum-breakers' used for normal paper production. The reel of paper is drawn across steel blades, or similar, under tension resulting in minute gaps between the particles of gum that allows the paper to expand or contract depending on climatic conditions without causing a curling of the substrate - the bane of postal counter clerks and collectors. On most stamps, this process is invisible to the eye when examining the gum on the issued stamp, but on some German, American and possibly stamps from other countries as well, there are colourless lines across the gum at intervals that serve the same purpose.
The Tonga gold-foil stamp substrate was too thick to pass across the steel blades without causing damage before printing, hence the new gum.
DAVAC was also used on two issues of stamps from Canada, but as these comprised normal thickness paper, it may explain why they did not continue with this gum type. However, it fails to explain why the Canadian printers ever bothered with DAVAC-gummed paper in the first place, as there are no obvious benefits when used on normal thickness paper