Let’s start with a tip. Kits sold in auto stores for polishing headlamps can also restore the clarity to portlights. If your plastic portlights are cloudy, not crazed, this is where you should start. Unfortunately, polishing won’t help aging portlights if they’re crazed or discolored. Eventually, the only way to restore a clear view to a plastic portlight is to replace the pane.
What material should you use? For longevity the hands-down winner is tempered glass—the 40-year-old opening ports on our boat are as clear as the day they were installed—but because glass is brittle it should only be installed in a rigid frame. That generally rules it out for fixed portlights, even those with metal frames, because such frames are not rigid, but flex to the contour of the cabin side.
For toughness, polycarbonate is best. Who does not think of Lexan, GE’s brand of polycarbonate, as “bulletproof?” That is not hyperbole, and polycarbonate’s legendary impact resistance makes it attractive for boats sailing in high latitudes. For the rest of us, however, polycarbonate is not such a good choice for fixed portlights. It is much more expensive than acrylic, but not any stronger. Its impact resistance is due to “give,” rather than strength, which means polycarbonate portlights are less likely to break, but more likely to breach their seal. Polycarbonate is not as clear as acrylic and soon suffers clarity-reducing UV damage in the marine environment unless you select the even more expensive treated types. It is softer than acrylic, so it is easily scratched, even by the wash of salt water. In short, except for impact strength, you pay more and get less when you choose polycarbonate over acrylic.
Sheet acrylic is sold under numerous brand names, Plexiglass being the best known. It is nearly always the first choice for replacement panes in fixed portlights, except for portlights with lots of curvature—significant bending creates internal stresses that lead to crazing. Cast acrylic is preferable, if you are willing to pay a small premium, but extruded acrylic is perfectly serviceable. By the full sheet, acrylic is much less expensive than polycarbonate, and may be cheaper still if purchased from a supplier’s “cutoff bin.” It is more readily available and comes in a wider variety of thicknesses and tints. Clear acrylic lets in more light below, but surface-mounted portlights are usually “smoked” to disguise the size and shape of the opening. If you are concerned about toughness, thicker acrylic is likely a better choice over polycarbonate replacements.
Before buying any new plastic, you should first dismantle one portlight to determine the dimensions of the original panes. Installing a thicker pane is often a prudent upgrade, but you need to dismantle a portlight to see if it is feasible. Surface-mounted panes of virtually any thickness can be accommodated without difficulty. For those sandwiched between exterior and interior frames, the maximum thickness would seem to be the separation distance, but it can be a relatively simple matter to create a spacer to match the part of the interior frame that rests against the cabin wall. Such a spacer can be used to expand the space between clamping frames, allowing for the installation of thicker panes.
Dismantling a portlight also exposes problems with fasteners, frames, pane shape and seal design. Older metal frames often have cast and threaded sockets that strip when dismantled. Reassembling them usually requires drilling through the socket and the frame and using through-bolts and cap nuts as fasteners. Worse still, aluminum frames may suffer from severe corrosion on the inside and can fracture when you pull them free from the cabin side. Replacement frames are rarely available, but can be fabricated either from sheet aluminum stock, or can be cast in bronze by a foundry using the original frame as a pattern. You might also discard the frame altogether in favor of surface mounting.
Dismantling a portlight can also reveal rubber gasketing, hard spacers, exposed or rotten core (which will need to be corrected before proceeding), panes that do not match the opening and absent or excessive sealant. All of these items need to be considered before deciding on the thickness and the shape and size of the replacement pane.
Cutting the Plastic
When the existing panes are the correct shape, the easiest course is to let your plastic supplier duplicate them in whatever thickness you specify. If you’re changing the shape or size of the panes, provide the fabricator with an exact pattern. If you are abandoning framed portlights for surface-mounted ones, making your patterns from black poster board will let you simulate the new “look.” Changing the corner radius or rake at one end can have a disproportionate aesthetic effect. Surface-mounted ports should extend not less than an inch beyond the opening they are covering.
If you want to fabricate the new panes yourself, you will need a variable-speed jigsaw with a carbide blade intended for plastics. The paper or plastic film on both sides of the material protects the surface from the shoe of the saw and allows you to mark your cut line easily and boldly with a fine-point marker. Leave this protective film in place until after the pane is completely installed.
For a framed portlight, simply follow your cut outline with the saw blade. Run the saw at a low speed. At the right speed, your saw will create acrylic chips. Run it too fast and the friction melts the plastic, causing slag to form behind the cut. Beeswax or even bar soap on the blade will help keep it cool.
Acrylic is notch-sensitive, which means it may fracture where there is a surface flaw. It is important to use a sharp blade and feed your saw smoothly to avoid creating micro-cracks along the cut edge. I always sand and polish even a hidden edge to eliminate this risk.
Cutting acrylic with a router is best left to professionals, but if you are duplicating a pane or you have a rigid pattern, shaping with a router can give good results. Cut a blank about 1/4 inch oversize all around, then center the blank on top of the old window or the rigid pattern and clamp or double-stick it in place. With a flush-cut bit in your router, set the depth for the guide to bear against the edge of the old pane or pattern, then run the router around the perimeter in a counterclockwise direction.
A router with a roundover bit is also the best way to remove the sharp edge of surface-mounted portlights. Rounding the outside edge enhances both safety and appearance. After rounding, finish-sand the entire edge, wet sand with 600-grit paper, then polish to a shine.
Two products you should never use to bed plastic portlights are polysulfide (Life-Calk, etc.), which attacks the plastic, and polyurethane (3M 5200, etc.), which is attacked by the plastic. One polyurethane, Sika 295 UV, can be used if you also apply a special primer, but this is more complicated and more expensive.
Another product to avoid is any methacrylate adhesive (MA590, etc.), even if this is how the portlights were installed originally. Because acrylic expands and contracts with temperature changes much more than fiberglass, a rigid bond is certain to suffer shear failure, and what doesn’t shear will be impossible to separate. Corrective measures are too awful to contemplate.
The sensible choice for bedding plastic windows is silicone. Unfortunately, most marine silicone sealants have marginal adhesive properties at best. This isn’t a problem for framed windows, but can be for surface-mounted portlights. In this case, the better choice is a silicone used to attach exterior glass to high-rise buildings, in particular Dow Corning 795 Silicone Building Sealant. Because this type of silicone is much more tenacious but no more expensive than other types, it is an equally good choice for framed windows.
Replacing framed portlights generally involves some rehabilitation of the frames. Tapped frames may need to be drilled out and reassembled with through bolts. Both the frames and the perimeter of the opening should be mechanically and chemically stripped of all old sealant. Any traces on either surface will interfere with the new seal. This is also the time to address exposed core issues.
With the frames clean and polished, the core sealed, and the perimeter of the opening squeaky clean, install the new frame in the portlight to check for fit. Between winter and summer, a two-foot plastic pane can lengthen by close to 1/8 inch, so the pane must not fit tightly. Screw the frame halves together temporarily, and if all looks right, break off a fresh blade on your razor knife and lightly (and carefully) trace it around the inside perimeter of both frame halves. You want to slice the film without scoring the plastic beneath. Now is also the time to mask off the perimeters of the frames and cabin sides. Remove the window and peel away the outer rings of film created by the razor cut, leaving the remainder of the pane still covered.
It is best to bed your new portlights in two stages. Put some tape across the face of the outside frame and the new pane to temporarily hold these together. Position this assembly in the opening and insert the fasteners. Inside the cabin, coat just the frame with a thick application of silicone sealant. With a helper outside, fit this frame and tighten the fasteners a little at a time in a “most opposite” sequence until some sealant squeezes out all around the inside and outside perimeters of the frame. Stop. With all sealants, especially with silicone, what you are trying to do is create a custom-fit flexible gasket.
Allow the sealant to cure for several hours to bond both the pane and the inside frame in place with the outside face of the pane perfectly flush with the cabin side. Unscrew the fasteners and remove the outside frame. Fill all voids between the pane and the opening perimeter with sealant, then apply a thick coat to the frame and reinstall it. Give each fastener a necklace of sealant, then with your helper inside, again tighten the fasteners incrementally in a most opposite sequence until you have a continuous bead of sealant squeeze out around the inside and outside perimeters. Stop.
A day later, run a fresh razor knife blade along the edges of the frames to cut away the excess sealant. Peel away the tape and the film and you are finished.
Surface-mounted portlights, like car windshields, are mostly glued in place these days. This is most secure when the pane fits into a rabbeted socket, but even retrofitted portlights sitting proud on a cabin side can be installed without mechanical fasteners. Without a frame, fasteners point-load the plastic and interfere with its free expansion. If bolts or screws are used in a surface-mounted installation, they are there just to hold the pane in place while the sealant cures. After that they are simply insurance against adhesion failure. As this is unlikely, especially in the sailing conditions most of us are likely to encounter, it is usually best to leave the fasteners off.
The alternative to mechanical fasteners is a super-strong, double-sided acrylic foam tape, which will both hold the pane in place and create a gap for the silicone sealant. The width of this gap is critical, as you want the pane to expand and contract without stressing the seal. The tape to use is 3M VHB 4991, which is 2.3 millimeters thick. Select 1/2in width.
Take great care to get the new pane properly aligned, then mark the cabin side around the entire perimeter of the pane. With the pane held tightly in this position by an outside helper, trace the perimeter of the opening from inside with a fresh razor knife, then peel away the ring of film your cut releases. Mask the edge of the pane, placing the tape flush with the pane’s now-bare inside surface.
If the color of your plastic is not dark enough to hide the grey tape (and sealant), you may want to paint the inside perimeter of the pane. This will also protect the adhesives from potential UV attack, but the bond strength will depend on the paint adhesion. According to some professionals, silicone-based, high-temperature exhaust paint is both compatible and tenacious. The area to be painted is the area exposed when you remove the ring of film.
VHB tape is sensitive to contamination, so just prior to applying it, wipe the exposed (or painted) surface of the plastic with a 50/50 mix of isopropyl alcohol and dry thoroughly. Apply the tape to the plastic first, because this allows you to control the width of the sealant application to come. Once in place, the gap between the fiberglass and the plastic pane will be just under 1/8in, which limits the desirable depth of the sealant. Position the tape 1/2in from the outside edge of the pane. This allows the silicone sealant to achieve maximum adhesion without unduly restricting thermal expansion. Cut and piece the tape at rounded corners to maintain a uniform distance to the edge. Press the tape firmly to the pane.
You should have already cleaned the involved fiberglass surfaces with acetone and perhaps toluene, but now wipe them again with your 50/50 mix of isopropyl alcohol and dry. Before removing the film from the tape on the pane, practice positioning it. Once you put the tape down you will be unable to adjust its position, so it is essential to work out the alignment in advance. Once your confidence is high, peel the film from the tape and oh-so-carefully align the pane, then press it to the cabin side. Now really push on the pane all around to maximize the bond.
This attachment is permanent, does not stress the plastic and leaves a uniform gap for the sealant. Mask the cabin about 1/8 inch beyond the perimeter of the pane, taking pains to match the radius of the corners. Load a sealant cartridge into an easy-to-operate caulking gun, clip the tip for a narrow bead, and pump the gap full of Dow 795, always moving the tip of the gun forward, never dragging it back. If necessary, push the sealant into the gap with a finger or a tool to fill it completely. Drag your finger along the edge of the pane to create a rounded fillet. When you are satisfied, peel away both the masking on the cabin and that on the edge of the pane. Clean up with alcohol.
All that remains is to remove the film on the acrylic and some kind of cosmetic treatment inside to hide the raw edge of the opening. This could turn out to be a good use for some of the VHB tape you have left over.
Enjoy the view.