PAINTS AND PAINTING. 



oil, so that the varnish simpiy holds the particles of pigment together in the same manner as the 

 rem in in a com red-. Tin-re an- two kinds of graphite in common use for paints the granular 

 ami ttu- llaki- graphite. 'I IK- Dixon Graphite Co., of Jersey City, uses a flake graphite combined 

 with silira, while tin- Detroit Graphite Manufacturing Co. uses a mineral on- with a Urge i* r- 

 (viu.tge of graphitic rarl><ui in granulated form. On account of the small specific gravity of the 

 pigment, carbon and graphite paints have a very large covering capacity. The thickness of the 

 out is, however, corn ->p mdingly reduced. Boiled linseed oil should always be used with carbon 

 pigments. 



Mixing the Paint. The pigment should be finely ground and should preferably be ground with 

 the oil. The materials should be bought from reliable dealers, and should be mixed as wanted. 

 If it is not possible to grind the paint, better results will usually be obtained from hand mixed 

 paints made of first class materials than from the ordinary run of prepared paints that are supposed 

 to have been ground. Many ready mixed paints are sold for less than the price of linseed oil, 

 which makes it evident that little if any oil has been used in the paint. The paint should be 

 thinned with oil, or if necessary a small amount of turpentine may be added; however turpentine 

 is an adulterant and should be used sparingly. Benzine, gasoline, etc., should never be used in paints, 

 as the paint dries without oxidizing and then rubs off like chalk. 



Proportions. The proper proportions of pigment and oil required to make a good paint 

 vary with the different pigments ; and the methods of preparing the paint; the heavier and the 

 more finely ground pigments require less oil than the lighter or coarsely ground while ground 

 paints require less oil than ordinary mixed paints. A common rule for mixing paints ground in 

 oil is to mix with each gallon of linseed oil, dry pigment equal to three to four times the specific 

 gravity of the pigment, the weight of the pigment being given in pounds. This rule gives the 

 following weights of pigment per gallon of linseed oil: white lead, 19 to 26 lb.; red lead, 25 to 33 lb.; 

 zinc, 15 to 21 lb.; iron oxide, 15 to 20 lb.; lampblack, 8 to IO lb.; graphite, 8 to 10 lb. The weights 

 of pigment used per gallon of oil varies about as follows: red lead, 20 to 33 lb.; iron oxide, 8 to 

 25 lb.; graphite, 3 to 12 lb. 



Covering Capacity. The covering capacity of a paint depends upon the uniformity and 

 thickness of the coating; the thinner the coating the larger the surface covered per unit of paint. 

 To obtain any given thickness of paint therefore requires practically the same amount of paint 

 whatever its pigment may be. The claims often urged in favor of a particular paint that it has a 

 large covering capacity may mean nothing but that an excess of oil has been used in its fabrication. 

 An idea of the relative amounts of oil and pigment required, and the covering capacity of different 

 paints may be obtained from Table VIII, Chapter XIII. 



Light structural work will average about 250 square feet, and heavy structural work about 

 150 square feet of surface per net ton of metal. 



It is the common practice to estimate J gallon of paint for the first coat and f gallon for the 

 second coat per ton of structural steel, for average conditions. 



Applying the Paint. The paint should be thoroughly brushed out with a round brush to 

 remove all the air. The paint should be mixed only as wanted, and should be kept well stirred. 

 When it is necessary to apply paint in cold weather, it should be heated to a temperature of 130 

 to 150 degrees F.; paint should not be put on in freezing weather. Paint should not be applied 

 when the surface is damp, or during foggy weather. The first coat should be allowed to stand for 

 three or four days, or until thoroughly dry, before applying the second coat. If the second coat 

 is applied before the first coat has dried, the drying of the first coat will be very much retarded. 



Cleaning the Surface. Before applying the paint all scale, rust, dirt, grease and dead paint 

 should be removed. The metal may be cleaned by pickling in an acid bath, by scraping and brushing 

 with wire brushes, or by means of the sand blast. In the process of pickling the metal is dipped 

 in an acid bath, which is followed by a bath of milk of lime, and afterwards the metal is washed 

 clean in hot water. The method is expensive and not satisfactory unless extreme care is used in 

 removing all traces of the acid. Another objection to the process is that it leaves the metal wet and 

 allows rusting to begin before the paint can be applied. The most common method of cleaning 

 is by scraping with wire brushes and chisels. This method is slow and laborious. The method of 

 cleaning by means of a sand blast has been used to a limited extent and promises much for the 

 future. The average cost of cleaning five bridges in Columbus, Ohio, in 1902, was 3 cts. per sq. 

 ft. of surface cleaned.* The bridges were old and some were badly rusted. The painters followed 

 the sand blast and covered the newly cleaned surface with paint before the rust had time to form. 



Mr. Lilly estimates the cost of cleaning light bridge work at the shop with the sand blast at 

 $1.75 per ton, and the cost of heavy bridge work at $1.00 per ton. In order to remove the mill 

 scale it has been recommended that rusting be allowed to start before the sand blast is used. One 

 of the advantages of the sand blast is that it leaves the surface perfectly dry, so that the paint can 

 be applied before any rust has formed. 



* Sand Blast Cleaning of Structural Steel, by G. W. Lilly, Trans. Am. Soc. C. E., Feb., 1903. 



