336 



PAINTS AND PAINTING 



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 

 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 square foot 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 sur- 

 face perfectly dry, so that the paint can be applied before any rust has 

 formed. 



Cost of Paint. The following costs of paints will give an idea of 

 costs and proportions used :** 



Oxide of Iron (Prince's Metallic Brown). One gallon of paint. 



6% Ibs. mineral at I cent .......................... 6 cts. 



Ibs. raw linseed oil 5-6 gallon at 56 cents ........ 47 ' 



Cost of materials per gallon of paint ................ 53 cts. 



*Sand Blast Cleaning of Structural Steel, by G. W. Lilly, Transactions 

 A. Soc. C. E., Feb., 1903. 



**Walter G. Berg, Engineering News, June 6, 1895. 



