Whenever I do any internal work on a watch, I always grease the o-ring/s. When waterproofing a watch, I use Marine epoxy to seal off the non-functioning He valves on reps.
On some watches that fail WR testing due to the crystal seal, I first remove crystal, clean all mating surfaces, replace gasket for new, and try again, if it fails once again, then I pop the crystal, and then apply a very thin layer of Resi-Bond to the gasket, then when crystal is pressed in, leave for 48 hours, then test again.