Liquid thread-lockers have become quite popular, because they reduce the parts inventory, are easy to apply, and low-cost. But they can be used carelessly, creating big problems for safety and EMC. Examples of liquid thread-lockers include ‘red’ and ‘blue’ products from Loctite and it has become quite common for people to use the word ‘Loctite’ to mean a chemical thread-locker. It’s like calling all vacuum cleaners ‘hoovers’.
The problem is that all liquid chemical thread-lockers are insulators, and because they have low viscosity and cover most or all of the thread, they ruin any electrical or RF bonding that a fixing might be supposed to provide.
A few years ago, I was working on a product that used a 4mm diameter screw thread as protective conductor (“safety earth”) connector. The entire protective conductor circuit is supposed to measure less than 0.1 Ohms, but this one screw connection alone was 8 Ohms! I noticed that it was coated with something red, which I presumed was a liquid thread-locker. After I scraped it all off, the fixing measured under one milliohm, instead of 8 Ohms!
Clearly, using liquid thread-locker on any screwed/bolted electrical connections is very bad, and quite possibly illegal and unsafe too.
It’s worth noting that there is a conductive liquid product made by Loctite, but it is an epoxy adhesive – not a thread-locker. I often hear people suggest using ‘conductive Loctite’, when what they really mean is conductive thread-locker, but as far as I have ever been able to discover no-one makes (or ever has made) conductive liquid thread-locking products. I imagine the addition of the necessary conductive fillers would make the viscosity too high for a thread-locking product.
Of course, a high resistance in any RF-bond will ruin it, just as surely as it will ruin an electrical safety connection.