I installed the compression ring on the lower part of the drain tube when it came in. Tested it for leaks at that point. All ok.

Couple days later started leaking again. Looked up at the donut connection  and seen leaks there. 

Today I took apart the drain tube at bottom of wash basin. Installed plumbers tape on threads and ran a bead of plumbers putty onto the top  of the compression ring contacting the drain tube. Assembled all that and installed the trap back in place.

Tested for leaks and did not see any.

Never had to do all that before having no leaks at that point. 

Quite possibly the casting on the wash basin had a flaw in it where the donut ring contacts. The threading on the plastic tube could not have been made to closer  tolerance.

Who knows as long as it does not leak.

Put the pail back under drain and will keep checking.