High voltage sealant coating
It sounds like you want to get some kind of conformal coating. There are several types- silicone and acrylic being the most common. Epoxy can be used, but it's about impossible to remove without destroying the (epoxy) PCB. 

Conformal coatings are available in aerosol cans for small-scale use. 

Do not use ordinary RTV silicone for electronics without carefully evaluating it.. much of the material on the market is cured with acetic acid, which is corrosive and conductive. Platinum (or tin) cure types are recommended for electronics use.