The paste that was around the head of the plug was very wet and mushy, like it just came out of the can....never got a chance to dry out.

The builder suggested maybe using 3M Weatherstrip Sealant which I assume is this stuff: 3M

I was also considering BRP gasket sealing compound used on many of the bolt threads on 2-stroke outboard motors. It looks like brown contact cement without being stringy going on. They say it's non-hardening, but it sets up solid where exposed to air. I've used it on outboard intake bolts that protrude into the crankcase. O.M.C. - OMC Gasket Sealing Compound

 

I wouldn't use the 3M stuff to seal that plug, it's basically just glue and not a sealer. I'll bet that the 2nd stuff would work, or even some RTV if in a pinch.