Hi Graham,
We replaced four Lewmar rectangular portlights three years ago (Lewmar replaced them when the powder coating failed prematurely). I was very concerned that the frames would be stuck with sealant to the GRP and therefore very difficult to remove. This was not the case, the frames are “sealed” to the GRP with black neoprene rubber seals which are easily removed and replaced. Portlight seals are readily available. I would council against sticking the frame to the GRP with “silicone” it will make any future work very difficult (as suggested by Steve C). The machine screws holding the inner and outer frames were countersunk on the inside and unfortunately cross head rather than “Torx”, being stainless in aluminium they were inevitably corroded and difficult to remove (I was surprised to see hex head bolts in your photo!). We replaced them with “Torx” and an anti corrosion barrier to facilitate any future work.
If the leak is between the frame and GRP the fix is easy. I am pleased to say that do not have any experience of dealing with leaks between the frame and acrylic.
Andrew
The membership association for owners of Dufour Yachts