Never use thread lock on spark plugs, there is nothing trying to unscrew the plugs once they are in place, all they have to do is seal in combustion gasses. The problem with most plugs is getting them back out to replace them and thread locker will make that a lot harder and may damage the aluminium thread in cylinder head, and even shear the thin steel wall of the plug between heaxagon and thread...
Modern Iridium / Platinum plugs of a decent brand ( NGK, Denso for example) have a special coating on the threads to prevent them sticking even after many years ( most of them are good for 70,000 miles ).
Do not be tempted to overtighten spark plugs 'to seal them better' as there is only a thin wall of steel between hexagon and thread and it can easily shear - then you really have a problem.......