It really shouldn't be necessary at all.

I'm assuming that this being the beginners' section we're talking glow engines. Also, I only expect throwing a nut to be a real possibility if the engine reverses very fast and hard... in normal starting and running the nut should be tightened enough to not allow any turning, so normal operation isn't part of the issue.

On a 2-stroke engine, the only time the engine does a fast reversal is if it's detonating -- which only happens if the ignition is too advanced. On starting, you can tell that's happenning by the engine preferring to start in reverse, taking a lot of turning to start, and either bumping around a lot on a starter or snapping back and hiting the top of your fingers or stick on a hand start. To retard the ignition, you usually need to turn down the current on your glowplug (if using a power panel, just turn the power down, if using a ni-starter/headlock/etc type, use a less fresh batter in it.) If the problem is once the engine's running, you can retard the timing by using a cooler plug, by reducing the compression (by adding a gasket or shim under the head), and/or by richening up the fuel mixture -- mixture being the most common cause. Once the engine's running right, there's nothing to work against the tightness of the nut and no reason to add anything to it.

Some 4-strokes are a bit balkier even when set up well, so they can need help keeping their props more than 2-strokes.

The approach, though, is usually to add another locking nut rather than using thread locking stuff. You may need to remove that nut fairly often to change or replace preops or to get the prop, spinner, etc out of the way of other things, so adding stuff to the threds each time may make it hard to get your prop off and on when you need to.