With small paint brush I have used:
Min wax oil based polyurethane fuel proof to 15% or so.
Pactra polyurethane of same or close color brushed into seam and then wipe excess off with rag.
Same with Top Flight Monokote paints.
With the spray can stuff I just spray some into a container then use the brush.
Sometimes thin super glue will wick down the seem, sometimes makes a mess. Must be brave.

Sig Stix it sometimes works sometimes not.
Still trying to figure out best way to use it.
The coverite brand stuff seemed to work better than the Sig.