Amen on the blue LocTite. I have used that stuff for many years when racing sports cars and for other odd needs like glasses screws.
They have multiple grades, the most common being blue, red, and green. Just to confuse things they all come in red bottles with just a big color dot to indicate contents. Buy the smallest tube or bottle that you can; the stuff is kind of nasty and tends to crawl out of its container over time. Put it in a ZipLoc bag for protection.
I have never had an issue removing glasses screws that I have loctited, but if you have any kind of issue, just hold a soldering iron on the head of the screw for maybe 10 seconds. Heat breaks down the bond and the screw should come easily.
Re other brands, I know that NAPA screw lock is simply LocTite with private branding. The NAPA stuff might be easier to find locally.
Re varnish, paint, etc. Why? That stuff is not designed for locking threads and there is an easily available product that is designed for the exact purpose. So IMO it is silly not to use it.