This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.
A writer in "Popular Mechanics" recently explained his method of overcoming the difficulty of making paint adhere to zinc, his desire being to make zinc be the support of a plate surface, and painting the surface with a paint composed of shellac in alcohol containing a solution of pumice stone and lampblack. The method employed was as follows:
A portion of the ' paint' was diluted with alcohol and the surface of the zinc well sand-papered while thoroughly wet with the solution-the intention being not to allow any of the surface to come in contact with the air during or after the operation of sand-papering. The paint was thick enough to cover thoroughly as it dried. When every part had been gone over thoroughly, a number of coats were added, in order that the low places might be filled up and the surface ground with pumice stone. Time has proved the success of this experiment. Many years have gone by, but the coating holds perfectly and the matter is now brought forward with the hope that the suggestion may have useful application in ordinary painting.
The theory on which this experiment was based is that zinc oxidizes instantly wherever a fresh surface is exposed to the air. This oxide is so thin that it is not visible. Nevertheless, it is present, and forms an unstable attachment for the paint as commonly applied, thus preventing the same from coming in absolute contact with the metal itself. The sand-papering operation, under cover of the wet paint, produces a different condition of things.