I put a large blob on the frag and when it is placed in the water it hardens the outside but not the inside.  
I put the frag where I want it and give it a few twists which breaks the skimmed seal, and hold it in place about 3 minutes. It almost always takes the first time. 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
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