Glow plug cylinder head insert sealing?

[robbocop33]

Just bought an old cls merc,changed the glow plugs and one broke, and yes,right at the rear!,in fact i think it was snapped/weak already near enough.
Got my trusty glowplug removal kit out,and for reasons unknown to me the initial step down drill bit went off centre causing the glow plug body not to come out,i then had to bite the bullet and go for the drilling out the head hole slightly larger and putting in an insert.
This went ok but there is 'slight' play between the thread i tapped and the insert.What is the best product to smear on the outer threads of the insert to ensure a gas tight seal,cold weld or something?Thanks