Can't get radiator bleed screw to seal.

I've got an old cast iron panel radiator that I've recently had stripped and repainted.
On piping up it has a weeping leak from the bleed screw at the top.

Taking the screw out and cleaning up I could see the screw thread has a notch in it and the hole that it screws into also has notches in the thread.

I tried a bit of Boss green jointing compound on the thread but that still leaked. Then tried loctite 55 but again it still weeped. Don't want to over tighten the screw as its soft brass and already starting to round off.

Beginning to think the thread is at fault, not seen one with a notch in before. Would this be why it is leaking?

Not keen on stripping the boss off and replacing with new as they are PITA to get off !

 

The thread etc are notched so when you open the valve, water or air can get out (otherwise it'd be working through thethread).
The actual seal is the tapered end of screw- clean it up a bit (and the seat in the boss) and you might be in luck. If not, a smear of boss white on the end should seal it til next time you use it...
EDIT Or maybe a small o ring...

 

There's a significant ridge on the cone on that bleed screw, hopefully the corresponding seat isn't so damaged.

https://www.bleedscrews.co.uk/

 

Try a new bleed screw.