Leaky portlights and hatches are one of the more frustrating projects to face on an old boat.
The best case scenarios are easiest to deal with, and these are usually the ones in which bedding has dried out and a simple removal, cleanup, and re-bed game plan is all it takes. When an acrylic (Plexiglas) or Lexan (polycarbonate) lens is removed, be very careful with solvents used to clean away old bedding because they can destroy the surface of once clear plastic.
To reattach the mechanically fastened lens, use a thick, adhesive butyl-rubber tape or equivalent bedding material instead of conventional tube-type sealants. (Practical Sailor testers have had good luck with Bomar hatch mounting tape.) Place the -inch-wide bedding on the lens like thick tape, and squeeze in the mechanical joint between the lens and the cabin house. It acts like a compressed grommet as well as an adhesive seal. Fastener holes drilled into the lens should be slightly oversize, never chamfered, and the fasteners coated with a sealant when installed.
In all too many cases, the leak is a symptom rather than a problem. The underlying cause likely is that the holes in the monocoque structure create a loss of stiffness, resulting in excess cabin house flex. Rig loads carried to chainplates, mid-boom sheeting arrangements, and genoa track-induced flex can cause significant deflection. Such deformational loading creates energy hot spots that can change shapes or even fracture laminate. In situations where stiffness increases dramatically-like where mechanical fasteners attach a port lens to a lengthy opening in a cabin coaming-a bending moment can twist the fastener(s), cracking the surrounding acrylic or polycarbonate.
The rigid, much more heavily laminated hull, lacks large apertures, such as holes for hatches, ports, and lockers, but the less structurally substantial deck laminate is Swiss cheesed with such penetrations. Better builders add extra reinforcement commensurate with the size and location of these holes, and it lessens the tendency for a port lens to be deformed by sailing loads.
In some cases, the problem can be solved by reinforcing the inside perimeter of the aperture with a stiff metal surround or additional laminate. Without addressing the structural problems that led to the leak, the drip, drip, drip will no doubt start again.