What would be the best way to electrically insulate the high voltage (60-90V) area on a PCB? I've seen a silicon-like coating on several PCBs, but I am not familiar with the product or its application method.
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.
Dielectric breakdown of air from surface contamination can drop from 1kV/mm to 100V/mm and even lower with corrosive salts used in "no clean" solder paste. Most plastic sprays or dips have limited but effective moisture barriers and have dielectric breakdown ratings ranging from 5-15kV/mm.
You might only need a 100um coating to improve breakdown in corrosive gas, flux or moist environments, and will want to consider speed of signals exposed to this dielectric coating which adds stray capacitance to nearby conductors.
The silicone that you see is usually just regular RTV silicone Here is an example by Permatex at permatex.com You can get it at any hardware store in either a tube like that in the hyperlink or a caulk tube to be used with a standard caulk gun. This particular one is rated to 400°F, just be careful not to insulate warm components like resistor bodies or heat sinks, etc.
This is also often used to reduce vibration on componets like connectors and capacitor bodies (lay the cap body down on the board and tack it in place with RTV).
You can also use epoxies or cyanoacrylate (super-glue), but RTV is fairly easy to work with.