In concert with John, my experience has been that when a very concientious operator applies too much coating "just to be sure", the coat skins over and either air, if it's under a component, or evaproating solvents or curing byproducts as here, can't escape and form a blister under the surface especially with heated drying.  If it's near a conductor, it can be a problem.  In either case, it's aesthetically undesirable.  Many operators and customers don't seem to realize that conformal coating is only meant to be a mil or two thick - it's not a potting.  If you've got problems with it puddling, maybe it has to be applied in a couple of thin coats.  But that's slower and more labor intensive - both bad concepts.

George 
(State of Franklin) 

>We've got a problem here, where we're hand spraying a urethane
conformal
>coat on over a rectangular inductor.
>The inductor body is a plastic tube, with potting to fill.
>The top is not always flat (sometimes concave).
>The problem is that bubbles appear in the conformal coat on the potted
>surface, sometimes right after bake and sometimes days after.
>
>Any ideas on what the root cause is and how to prevent the bubbles?
>

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