Threaded coolant hose leaking...what to seal the threads with?

Are you sure you need thread sealant?

If it's a flare fitting thread sealant isn't going to help

Might need to clean the mating surfaces or replace the fitting if it's chewed up

 

I use a thread sealant from Home Depot.

I can't remember the name of it off the top of my head but it's the white paste, not the brown.

I have found that to work 100% of the time and it is an excellent product.

In a pinch I will use teflon tape, but usually that's just for Airlines and such.

Home Depot also sells a tool that is like a small wire brush shaped in a way that's made to clean threads.