MIL-I-46058 & now IPC-CC-830 say conformal coatings should be "bubble free". Some have interpreted this as meaning that a few small bubbles away from components are acceptable.
  Foam is not acceptable.
   
  Some polyurethane conformal coatings generate gas as they cure. They should not be coated on very humid days. 
   
  Bubbles are also apt to form in conformal coatings when the coating is too thick. I've seen this happen in between closely spaced tall components even when the coating on most of the board is within customary limits and has no bubbles. The solution was to change the application technique.
   
  Get the coating and equipment manufacturers involved. They want your future business.
   
  Ed Mines

       
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