there have been many threads about the difference between fastening and sealing (ie if you have mechanical fasteners - screws / bolts, then you can get away more with a sealant, from a gun or something like butyl tape - Mainsail / Halekai has lots of nice pics and comments in his posts )
I choose, as many are doing now, to use the VHB tape, as the mechanical fastener, instead of screws / bolts that are prone to leak. I added the Dow 795, which is both a strong adhesive (mechanical fastener / glazing silicon) as well as a "stretchable" sealer, that will stay adhered to both surfaces as they elongated or shrink at different rates.
I can tell you that I put in one portlight / deadlight by mistake, and had to remove it seconds later. The Dow 795 was obviously not cured yet, so the only adhesion was the VHB tape, which I had not even pressed on hard. It was all I could do to use my whole body weight to push it out. Now that they have cured, I doubt they could be pushed out - maybe after many hard kicks!!