Crevices may accelerate corrosion and are difficult to coat. 

 Continuous welding is more costly than skip welding, but it eliminates 

 such crevices. Riveted and bolted connections can also produce crevices. 

 Insulation not only minimizes crevices but may also eliminate galvanic 

 corrosion associated with contact of dissimilar metals. 



Guy lines should be placed so they do not contact each other (Figure 

 9) or structural members during high winds. The use of protective 

 sleeves to prevent abrasion damage by such action is not an acceptable 

 method of preventing contact (Figure 10). 



Surface Preparation for Coating Repair 



Antenna towers and supporting equipment are best coated in a steel 

 fabrication shop and then touched up later in the field, as necessary. 

 Because of the difficulty and cost of coating assembled antenna towers, 

 it is best to obtain as high a level of surface preparation and as good 

 a coating as possible to start with to forestall future maintenance as 

 long as possible. Abrasive blasting, as with almost all steel structures, 

 is the preferred method of cleaning steel towers for coating, either 

 before or after erection, because it produces a good surface texture for 

 bonding (Ref 5). The different levels of surface preparation commonly 

 used for new steel tower construction are given in Table 6. 



The level of surface preparation desired depends upon (1) the type 

 of coating to be used, (2) the severity of the environment, and (3) the 

 length of protection desired. Exterior abrasive blasting is being 

 restricted in some locations because of air pollution caused by particu- 

 lates emitted into the atmosphere. Thus, greater use may have to be 

 made of different methods of mechanical cleaning (Figures 1(b), (c), 

 (d)). The Tri-Services Painting Manual (Ref 6) describes such cleaning 

 methods as sand and power tools (brushes, grinders, sanders, hammers, 

 chisels, and scalers) and flame and chemical cleaning. Reference 7, 

 published by the Steel Structures Painting Council (SSPC), has standards 

 for hand-tool cleaning (SSPC-SP No. 2) and for power-tool cleaning 

 (SSPC-SP No. 3). Flame and chemical cleaning are not usually practical 

 on tower structures. 



Galvanized steel and aluminum are solvent-cleaned, if new, and then 

 hand-tool-cleaned or brush-off-blasted (SSPC-SP No. 4) before coating. 



Coating Selection 



Coating selection is based on the properties of the coating system; 

 some of these are discussed below. 



1. Unmodified drying oil coatings wet steel surfaces very well, 



but cure very slowly and lack toughness and durability for tower coatings. 



2. Alkyds (modified drying oil coatings) wet steel surfaces well, 

 have good curing and protective properties, and are also rather tolerant 

 of incompletely prepared surfaces. 



