tended period of time. It should, therefore, be covered with the finishing coat as 

 soon as possible after application. 



Although plastic is generally strippable and easily removable, it becomes a more 

 or less permanent coating when properly applied with binding agent forming a 

 base, and will withstand a considerable amount of rough treatment. 



Plastic material shrinks as it dries and tends to shrink further with aging. The 

 amount of binding agent used should be in relation to the size of the opening to 

 be covered and in no instance should it be applied in a band less than 2 inches 

 wide around the perimeter of the area to be sealed. This method of application 

 will prevent shrinkage from pulling the plastic away from the surface and thereby 

 breaking the seal. Where sealing is required in a corner where two surfaces 

 adjoin, such as is found under the eaves of the building, it is essential that a strip 

 of binding agent be applied directly in the apex of the angle, as well as on the two 



PMA 175, 



Figure 13.— Application of webbing compound on wide cracks around the edge 

 of a roll-down steel fire-door. 



margins, in order to keep the shrinkage from drawing the material across the 



corner. 



Use of Webbing 



Webbing compound is needed on any crack in excess of % inch; its application 

 is demonstrated in figure 13. On an opening or a void in excess of 18 inches, it 

 is necessary to break the opening into smaller sections by the use of strips of tape. 

 Webbing will bridge a space 18 to 24 inches wide, but beyond that point applica- 

 tion without support is not practical. As an illustration, a window 36 inches wide 

 by 72 inches high would require one center strip from top to bottom with three 

 cross strips evenly spaced. 



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