I have heard about the technique of spraying conformal coating to hot boards several times; one of my colleagues used to recommend it to prevent flow.
   
  The problem I always saw is that components loose heat at different rates so any heat related rate will be different on different parts of the board. Unless the board is in a heat supplying fixture or the process is completely automated there will be differences from board to board.
   
  FR4 contains a lot of glass which has poor heat retention. Metals on the other hand will stay hotter longer. 
   
  If a thick layer of coating is applied to a hot board by either dipping or flow coating bubbles will be prone to form.
   
  Ed Mines

       
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