516 ENGINEERING MATERIALS. CHAP. XV. 



Priming or Shop Coat. Engineers are very much divided as to what makes the best priming 

 coat; some specify a first coat of pure linseed oil and others a priming coat of paint. Linseed oil 

 makes a transparent coating that allows imperfections in the workmanship and rusted spots 

 to be easily seen; it is not permanent however, and if the metal is exposed for a long time the oil 

 will often be entirely removed before the second coat is applied. It is also claimed that the paint 

 will not adhere as well to linseed oil that has weathered as to a good paint. Linseed oil gives better 

 results if applied hot to the metal. Another advantage of using oil as a priming coat is that the 

 erection marks can be painted over with the oil without fear of covering them up. Red lead paint 

 toned down with lampblack is probably used more for a priming coat than any other paint; the 

 B. & O. R. R. uses 10 oz. of lampblack to every 12 Ib. of red lead. Linseed oil mixed with a small 

 amount of lampblack makes a very satisfactory priming or shop coat. 



Without going further into the controversy it would seem that there is very little choice between 

 linseed oil and a good red lead paint for a priming coat. For data on the standard shop paints 

 specified by different railroads, see digest of specifications in Chapter IV. 



Finishing Coat. From a careful study of the question of paints, it would seem that for ordi- 

 nary conditions, the quality of the materials and workmanship is of more importance in painting 

 metal structures than the particular pigment used. If the priming coat has been properly 

 applied there is no reason why any good grade of paint composed of pure linseed oil and a very 

 finely ground, stable and chemically non-injurious pigment will not make a very satisfactory finish- 

 ing coat. Where the paint is to be subjected to the action of corrosive gases or blasts, however, 

 there is certainly quite a difference in the results obtained with the different pigments. The 

 graphite and asphalt paints appear to withstand the corroding action of smelter and engine gases 

 better than red lead or iron oxide paints; while red lead is probably better under these conditions 

 than iron oxide. Portland cement paint or coal tar paint are the only paints that will withstand 

 the action of engine blasts. 



To obtain the best results in painting metal structures therefore, proceed as follows: (i) pre- 

 pare the surface of the metal by carefully removing all dirt, grease, mill scale, rust, etc., and give 

 it a priming coat of pure linseed oil or a good paint red lead seems to be the most used for this 

 purpose; (2) after the metal is in place carefully remove all dirt, grease, etc., and apply the finishing 

 coats preferably not less than two coats giving ample time for each coat to dry before applying 

 the next. The separate coats of paint should be of different colors. Painting should not be done 

 in rainy weather, or when the metal is damp, nor in cold weather unless special precautions are 

 taken to warm the paint. The best results will usually be obtained if the materials are purchased 

 in bulk from a responsible dealer and the paint ground as wanted. Good results are obtained with 

 many of the patent or ready mixed paints, but it is not possible in this place to go into a discussion 

 of their respective merits. 



ASPHALT PAINT. Many prepared paints are sold under the name of asphalt that are mix- 

 tures of coal tar, or mineral asphalt alone, or combined with a metallic base, or oils. The exact 

 compositions of the patent asphalt paints are hard to determine. Black bridge paint made by 

 Edward Smith & Co., New York City, contains asphaltum, linseed oil, turpentine and Kauri gum. 

 The paint has a varnish-like finish and makes a very satisfactory paint. The black shades of 

 asphalt paint are the only ones that should be used. 



COAL TAR PAINT. Coal tar paint is occasionally used for painting gas tanks, smelters, and 

 similar structures that receive rough usage. Coal tar paint mixed as described below has been 

 used by the U. S. Navy Department for painting the hulls of ships. It should give satisfactory 

 service where the metal is subject to corrosion. The coal tar paint is mixed as follows: The pro- 

 portions of the mixture are slightly variable according to the original consistency of the tar, the 

 use for which it is intended and the climate in which it is used. The proportions will vary 

 between the following proportions in volume. 



Coal Tar. Portland Cement. Kerosene Oil. 



New Orleans Mixture 8 I I 



Annapolis Mixture 16 4 3 



The Portland cement should first be stirred into the Tcerosene, forming a creamy mixture, 

 the mixture is then stirred into the coal tar. The paint should be freshly mixed and kept well 

 stirred. This paint sticks well, does not run when exposed to the sun's rays and is a very satis- 

 factory paint for rough work. The cost of the paint will vary from 10 to 20 cts. per gallon. The 

 kerosene oil acts as a drier, while the Portland cement neutralizes the coal tar. 



If it is desired to paint with oil paint a structure which has been painted with coal tar paint, 

 the surface must be scraped and all the coal tar removed. 



CEMENT AND CEMENT PAINT. Experiments have shown that a thin coating of Portland 

 cement is effective in preventing rust; that a concrete to be effective in preventing rust must be 

 dense and made very wet. The steel must be clean when imbedded in the concrete. There is 

 quite a difference of opinion as to whether the metal should be painted before being imbedded or 



