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Refitting hatch lenses
By Bob
Pingel, featured in Sailing Magazine
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Most deck hatches consist of an aluminum (or stainless steel) frame with a
plastic lens. The lens is typically held into the frame with a silicone-like
sealant. The sealant is typically laid in a wide bead (1/4-inch to 3/8-inch)
to firmly hold the lens in place and to allow the lens and frame to expand
and contract at different rates. Over time, the plastic lens will show its age. The first thing to go is the outside top surface—it will get scratched and dinged with everyday wear and tear. The surface can also become damaged from contact with harsh cleaners. Next, you will start to see internal crazing. The crazing shows up as tiny fractures inside the plastic; these fractures especially show up in the sun. Some of the crazing is from external impacts with winch handles, shackles and the like, but most of it is thermal. Thermal crazing occurs on a hot day with cool water. A dark plastic hatch will heat up to more than 100 degrees on a bright, sunny day. When cool water splashes on the hot hatch it can fracture. Finally, the color of the plastic will start to fade and change from exposure to the sun. All of that crazing and chipping is not only unsightly—it is also a safety issue. Plastic hatch materials are notch-sensitive. This means that once they are fractured or scratched that damage can more easily grow into a full-fledged crack. That ugly, scratched-up lens could end up collapsing under the weight of a wave crashing on your foredeck. The types of material for hatch lens replacements are regularly misunderstood. There are really only two types of plastic used, but they are sold under several different trade names. The two materials are acrylic and polycarbonate. Acrylic is sometimes sold under the trade name Plexiglas, and polycarbonate is sold as Lexan. The factors to consider when choosing a material are scratch resistance, chemical resistance, UV life and impact resistance. Acrylic has better scratch and chemical resistance, and far better UV resistance, but is not as strong as polycarbonate. Polycarbonate wins for absolute strength, but acrylic is a far better material for most boats. Polycarbonate gets its impact resistance from slight flexibility; large, unsupported polycarbonate hatches can sag where acrylic hatches will not. Unless the boat operates in an environment where impacts are likely, you will get far better performance from an acrylic hatch. As an added benefit, acrylic is much less expensive than polycarbonate. You will not find acrylic or polycarbonate of appropriate thickness at your typical corner hardware store—you will need to visit a plastics supply house Both acrylic and polycarbonate are available in different thickness, but strangely they are not typical fractional-inch sizes or metric. The sizes are close to even millimeters, but the material varies a bit in thickness. Your best bet, if you can’t get a perfect fit, is for the material to be a little too thick so that it stands proud of the frame. If it is too thin the lens will puddle water.
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Removal The first step is to remove the old lens, which can be done simply with a utility knife. Just cut around the perimeter of the lens from the top and the bottom. Once the bead of sealant is cut, we can pry the lens out of the frame with a stiff putty knife. |
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Sealant removal Removing the residual sealant is the hardest part of this job. The entire residue of the old sealant needs to be removed or the new sealant will not adhere to the frame. I don’t have any magic here; it will take a combination of scraping, wire brushing, sanding and solvents—not to mention lots of elbow grease and determination. Start with a utility knife to get the big chunks off. Next, scrape with a paint scraper. A wire brush with mineral spirits or citrus solvent will get the last of the sealant residue off. Finally, a light sanding will clean things up. To keep the little of bits of sealant and drips of solvent from getting into the cabin, line the open hatch with a plastic bag, and trap it between the deck and the hatch frame.
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Cutting the blank Next, fabricate the lens. This step will require a router and a flush cutting bit. If you do not have a router or the skills to use one, you can get the lens cut by a plastic fabricator. Use the old lens as a pattern for the new one. The first step is to remove all of the residual sealant from the old lens by scraping it with a utility knife. A fine film of sealant residue can remain, but you need a smooth surface to use as a template for the new one. Next, trace the outline of the old lens onto the masking paper of the new plastic. To make the router cut easier and cleaner, remove the excess material outside the traced line. Cut within a half-inch outside of the traced line with a saber or band saw. After the panel is rough cut, adhere the old panel to the new with double-sided tape. The tape will hold the two pieces of plastic together so that you can cut the new panel with the flush cutting bit. A flush cutting bit has a cutting surface and a router bearing. Run the router bearing on the old lens and the cutting surface will cut the new lens to the exact shape of the old lens. Practice with a scrap piece of plastic before you cut the real blank—acrylic needs a slow steady feed rate so that it will cut smoothly. The router will leave a slightly rough surface on the acrylic, but this “tooth” will help the glue to adhere.
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Masking Like any sealant job, you need to mask off the areas that you don’t want sealant on. Start by masking off the outside surface of the frame, masking right up to the edge. There is really no need to mask the inside of the frame, as you won’t get much squeeze out on the inside. Next, peel the masking paper off the top of the new lens and mask off an inch or so around the perimeter. The masking tape will be easier to remove than the original masking paper later when gluing the lens in. Peel off all the masking on the back (inside) of the lens. Again, you won’t get much squeeze out on the inside, and if you do, remove it after the sealant dries. Priming and gluing There are several sealants that could be used, but I have had the best results with Sikaflex 295-UV. This is a black polyurethane sealant designed as a direct glazing adhesive for acrylic lenses. It hardens to a firm yet flexible flat black, rubber-like material, much like the material used by the manufacturers. The sealant requires a cleaner, Sika-Cleaner 226, and a primer, Sika-Primer 209. The cost of the sealant, plus the cleaner and primer, make it relatively expensive, but you will be happy with the finished result. Sikaflex products can be hard to find, but many large chandleries carry them. If you can’t find them locally, a good mail order source is Jamestown Distributors in Bristol, Rhode Island. The first step is cleaning the gluing surface on the frame with the cleaner. Just wipe the surface with a rag dipped in 226, turning the cloth often. Once the surface is dry, brush on a thin coat of Sika-Primer 209. This primer looks like a thin black paint, so it will be easy to see where the primer is applied. Now that the frame is ready, move on to the lens. Similar to the frame, clean the edge and the bottom quarter inch or so of the lens and apply the primer to the edge and bottom of the lens. Be sure not to primer too far out on the bottom of the lens or the black primer will be visible. The gluing process is simple. Start by applying a bead of sealant on the bottom of the frame, and with the glue in place, drop the lens into the frame recess. Don’t worry about the minor squeeze-out on the inside of the hatch. Now, fill the void around the lens with sealant. Just pump in sealant all around the lens and dress it flat with your finger (preferably covered in a rubber glove). Once the sealant is smoothed out, remove the masking tape. You could wait until the sealant dries, but I find it easier to remove the tape now. Some of the masking tape may get trapped under the sealant if it’s allowed to dry.
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Clean Up After the sealant is dry (overnight in warm conditions, or longer in cooler conditions), you need to do a final cleanup. The first step is to clean up any squeeze-out on the inside of the hatch with a razor blade, being careful not to scratch the lens. Once the sealant is cleaned up, a spritz of cleaner will finish the job. By following these few easy steps, you too can wake up to see bright, shining mornings through your deck hatches. |