Originally Posted by Vapor
I only use dielectric grease for creating an airtight seal with the gasket foam (the foam ring between the mounting plate and the insulation on the board), where cold is not an issue at all (since that's after the thermal insulation steps) and in the socket, where cold is good and water is bad (this just fills the space, nothing else really--optional step, IMO, but I've always done it for DICE, so why not this?).
As for materials, play around on McMaster some with the keywords I've listed above....I won't give away all the exact products (with budget constraints especially, I bought large quantities, so my products will be slightly different, but I bought about what I thought would be enough for a dozen boards and spent just over $100, but turns out I'll need more lacquer every 2-3 boards :(), because if I say X, people will go get X when there might actually be a Y out there that either costs less or is better suited for the job, so just buy what you think you will be able to work with, what fits your budget, and makes sense to you and if it works GREAT, come back and let us know :)