I have a 1988 432 which is similar to the Oceanis 430. The long side windows are bedded in sealant. I just replaced all of the ports hatches and windows. Do not use screws if you don't have to as they will crack the acrylic over time.
I have a 1988 Beneteau First 285 I replaced the long Port and Starboard windows, with 1/4 inch dark Lexan, as well, the Large round one in the back 2 yrs ago and have not had any problems since with leaking. Our temperatures fluctuate here in Canada -40c to 30c, so in my mind what I did was a great success. I also reused the series screws with no problems.
1. clean area well
2. I purchased a couple of rolls of soft weather stripping approx. 1/4 inch size or smaller, sticky one side as to work with. what I did was, run the stripping all the way around the fiberglass side on boat, next do it again leaving approx 1/4 inch space between as to create a channel.
3. Caulk the channel you made to be even with the stripping, go right over the holes for the screws.
4.carefully take your pre-cut, pre-drilled Lexan, place on screw in one corner, and another screw in the opposite far corner, Have a helper for this but place gently the glass in its place lining up the screws to their respective holes. Have the helper go below and place the other part of the screw on but do not tighten as to apply any pressure. Continue to place all screws back, again no pressure.
5. Once all is in place, you can slowly tighten using a similar pattern to tightening a tire, (kinda like a cross pattern) (even tightening) you only turn 1/8th a turn a time, all the way around, from top side as to watch the caulking to meet the window without squishing out of the Gasket/weather stripping that was placed earlier. once satisfied the contact all the way around is there, lightly tap (not push on) the window with your fingers to ensure the seal is upper and lower. you're done, allow to dry, and once dry do not tighten or turn screws. this will only separate the silicone from the screw.
This is important and take note, that the window will not once done, be tight against the fiberglass, which is a good thing. (No cracking of glass down the road).
