Dont use "normal" silicon sealant. That vinegar smell you smell when you apply silicon sealant? That's acetic acid, its corrosive, bad for electronics... If you can find an electronics grade silicon sealant (neutral cure) thas OK (it wont have the vinegar smell). But honestly, I cant see any kind of liquid rubber working out well for an ESC since the rubber will hold in heat and thats bad for ESC's...
many people have had good luck with a product called Corrosion-X... its a spray thats made to stop corrosion... but works great to waterproof electronics without a thick coating and the associated heat buildup...
Here is a silicon sealant made for electronics http://www.mgchemicals.com/products/422a.html (click on "where to buy" at the top if you want to buy some...