I have had some success using Conformal coating gel.  One example is 1A33 Gel.  Be advised that it is expensive, nor does not come in user friendly packaging.
More often, if you customer will abide by it, is the application of a viscous adhesive such as Hysol 0151.  This also can serve as a hold-down/part reinforcement.
If you are NADCAP, please be advised, that until they make a correction, the use of Silicon backed masking dots is actually a violation of the standard.