PRESERVATION OF METALS USED IN MARINE CONSTRUCTION. 159 



How to paint involves the question of the proper preparation of the sur- 

 faces, the proper preparation of the paint, and a proper spreading rate of the 

 kind of paint used. 



When to paint involves the question of the dryness of the surfaces and 

 the atmospheric conditions in their vicinity. What kind of paint to use 

 involves the question of whether the paint is to be used (i) as a protec- 

 tion to the metal (anti-corrosive and waterproof paint), (2) as a cleanser 

 of the paint film of marine growth (anti-fouling or poisoned paints), (3) as 

 a decoration to the structure (covering and coloring paint). In any of 

 the above cases the paint film when dry should have about the same coeffi- 

 cient of expansion as the metal to be covered, or the painting will crack or 

 become loosened from the surfaces, and any good effect, even unto decoration, 

 will be lost. An anti-corrosive paint is one that, when well dried, is imper- 

 vious to moisture and keeps the metallic surface dry so that no active cor- 

 rosion can take place, and one that, when moisture does percolate through it 

 to the metal, dissolves and raises the potential of the enclosed water to a 

 point as high or higher than that of any metal exposed to it. To perform 

 this last function the pigments of the paint must be higher in the electromo- 

 tive series than the metal to be covered, and to perform the first the pigments 

 and solvent must form a homogeneous film over the whole of the covered 

 metal. An anti-fouUng paint is one that, when any marine growth sticks on 

 its surface, either poisons the growth and allows it to drop off or dissolves 

 under the growth, due to its secretions, and allows it to loosen and float away. 

 Decoration or distinguishing paints are those used to decorate the living 

 quarters or to distinguish different kinds of piping. About the only require- 

 ment for them is that they must have a pleasing or distinctive color and the 

 color must last as long as possible under the conditions of the service to which 

 they are exposed. Many good anti-corrosive paints are not good water- 

 proof ones ; therefore, unless the anti-corrosive paint is a good waterproof one, 

 it should be covered with a good waterproof paint, as the protective effect 

 of a non-corrosive paint is obtained at the expense of the paint unless it is 

 also a good waterproof one. As soon as enough of the paint is dissolved off 

 to enable a free circulation of water to the surface of the metal the anti- 

 corrosive effect is lost. 



Proper painting is a most important aid in the preservation of metals 

 and no effort should be spared to obtain it. The first cost of painting 

 for the protective effect must be disregarded and the best paint obtain- 

 able for the purpose used and apphed with the greatest care. There is 

 a general tendency in some directions to regard painting as only for decora- 

 tive purposes and to consider that the cheap paints frequently applied 



