The method used by the British navy yards for waterproofing and painting canvas so it will not become stiff and cracked is as follows:
One ounce of yellow soap and 1/2 pt. of hot water are mixed with every 7 lb. of paint to be used.
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Excerpt from the book: THE BOY MECHANIC VOLUME I 700 THINGS FOR BOYS TO DO WITH 800 ILLUSTRATIONS 1913, BY H. H. WINDSOR CHICAGO POPULAR MECHANICS CO. PUBLISHERS
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The mixture is applied to the canvas with a brush.
This is allowed to dry for two days and then a coat of the same paint, without the soap, is laid on.
When this last coat is dry the canvas may be painted any color desired.
After three days of drying the canvas may be folded up without sticking together, and is, of course, waterproof.
Canvas waterproofed in this manner makes an excellent covering for portable canoes and canvas boats.
The color mixture for the soap and second application is made from 1 lb. of lampblack and 6 lb. of yellow ocher, both in oil; the finish coat may be any color desired. When no paint is to be used on the canvas it may be waterproofed with a mixture made from soft soap dissolved in hot water, and a solution of iron sulphate added.
Iron sulphate, or ferrous sulphate, is the green vitriol. The vitriol combines with the potash of the soap, and the iron oxide is precipitated with the fatty acid as insoluble iron soap.
This precipitate is then washed, dried and mixed with linseed oil.