My Good Old Boat - Resealing Portholes

[ocarina]

Hello to eveyone at Cruisersforum. I'm a new member who came aboard looking for a few pointers. My 10 yr. refit of Ocarina II, a Douglas 32 continues this season with the replacement of the main portlights. This is the second time I've done this job and I think I know the right way now. I thought I knew the right way last time, but cracks appeared around the edges after a couple of seasons. This time I've allowed 1/6" per foot of space for expansion around the perimeter and will not be cranking the fasteners tight until the silicone sealent is mostly set. Last time I used sika which didn't work that great. After 10 yrs. the areas where the sika was squeezed thin it had the consistency of adobe although it remained gooey in the spots that were deeper than about 1/16". I also suspect that I had used extruded rather than cast acrylic back then as the condition of the material in the area near the fasteners appeared flaky like mica. I considered butyl tape instead of 3-m silicone but as my ports are inset with bronze bezels the void around the edge varied too much for tape to fill in evenly. This time I'm also washing all surfaces with acetone because the old sealent seem to bond inconsistently to the bronze and gelcoat. There was probably some oil contaminating the surfaces previously.
I hope this time I've got it right,but if anyone has any other advice,please feel free to share.