I'm replacing the forward hatch on my 25.5 this weekend. It is pretty much a direct replacement hatch in that it is made from molded plastic. My question is what should I use as a bedding compound? I've heard that 5200/4200 will soften plastic, and 5200 is a "permanent" type sealant. I was going to use Dow Corning 3145 RTV, unless someone has a better idea.
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Are you sure?
Are you sure it is "plastic"? If so then your choice of a silicone is appropriate. Usually the manufacturer recommends the proper sealant. If it is a compound that does not require silicone then you are far ahead to use a polysulfide, NOT a polyurethane. Here is the little reminder page that I keep on file: Sealants Polysulfides You can—and should—use polysulfide to bed almost everything. A synthetic rubber with excellent adhesive characteristics, polysulfide is the most versatile of marine sealants. As a bedding compound it allows for the movements associated with stress and temperature change, yet maintains the integrity of the seal by gripping tenaciously to both surfaces. It even adheres to oily teak and is unaffected by harsh teak cleaners, making it the choice for bedding teak rails and trim. It is also an excellent caulking compound since it can be sanded after it cures and it takes paint well. (The black caulking between the planks of a teak deck is invariably polysulfide.) "3M 101", "Boat Lifes’s Life-Caulk". One caution: do not use polysulfide to bed plastic—as in deadlights, portlight frames, or deck fittings. The solvents in polysulfide will cause acrylic, polycarbonate, ABS, and PVC to harden and split. Only when you know for certain that a plastic fitting is made of epoxy, nylon, or Delrin can you safely bed it with polysulfide. Below-the-waterline through-hull fittings fall into this group, but if you have any doubt, use another sealant. Polyurethane Consider polyurethane an adhesive rather than a sealant. Fittings bedded with polyurethane typically cannot be separated without damage, so do not use it on anything you might need to dismantle in the future. Polyurethane is an excellent sealant for hull-to-deck and hull-to-keel joints and a good choice for through-hull fittings, rubrails, and toerails. Do not, however, bed teak rails with polyurethane because teak cleaners damage it. Like polysulfide, polyurethane should not be used on acrylic, polycarbonate, PVC, or ABS-based fittings. "3M 4200", 3M 5200" Silicone If you think of silicone as a gasket material instead of a sealant you can probably intuit its appropriate uses. It is the best choice for bedding components that must be periodically dismantled. Its excellent insulating properties make it ideal for bedding dissimilar metals—stainless hardware on an aluminum spar, for example. And it is—by default—the only one of the marine sealant trio than can be safely used to bed plastic. However, silicone should not be used below the waterline. And because it depends upon mechanical compression to maintain its seal, silicone is also a poor choice for sealing hardware on a cored deck. Keep this quick review in mind when you’re considering a bedding compound: Polysulfide—a sealant suitable for bedding everything except plastic. Polyurethane—an adhesive that forms a permanent bond. Silicone—a gasket material and electrical insulator.
Are you sure it is "plastic"? If so then your choice of a silicone is appropriate. Usually the manufacturer recommends the proper sealant. If it is a compound that does not require silicone then you are far ahead to use a polysulfide, NOT a polyurethane. Here is the little reminder page that I keep on file: Sealants Polysulfides You can—and should—use polysulfide to bed almost everything. A synthetic rubber with excellent adhesive characteristics, polysulfide is the most versatile of marine sealants. As a bedding compound it allows for the movements associated with stress and temperature change, yet maintains the integrity of the seal by gripping tenaciously to both surfaces. It even adheres to oily teak and is unaffected by harsh teak cleaners, making it the choice for bedding teak rails and trim. It is also an excellent caulking compound since it can be sanded after it cures and it takes paint well. (The black caulking between the planks of a teak deck is invariably polysulfide.) "3M 101", "Boat Lifes’s Life-Caulk". One caution: do not use polysulfide to bed plastic—as in deadlights, portlight frames, or deck fittings. The solvents in polysulfide will cause acrylic, polycarbonate, ABS, and PVC to harden and split. Only when you know for certain that a plastic fitting is made of epoxy, nylon, or Delrin can you safely bed it with polysulfide. Below-the-waterline through-hull fittings fall into this group, but if you have any doubt, use another sealant. Polyurethane Consider polyurethane an adhesive rather than a sealant. Fittings bedded with polyurethane typically cannot be separated without damage, so do not use it on anything you might need to dismantle in the future. Polyurethane is an excellent sealant for hull-to-deck and hull-to-keel joints and a good choice for through-hull fittings, rubrails, and toerails. Do not, however, bed teak rails with polyurethane because teak cleaners damage it. Like polysulfide, polyurethane should not be used on acrylic, polycarbonate, PVC, or ABS-based fittings. "3M 4200", 3M 5200" Silicone If you think of silicone as a gasket material instead of a sealant you can probably intuit its appropriate uses. It is the best choice for bedding components that must be periodically dismantled. Its excellent insulating properties make it ideal for bedding dissimilar metals—stainless hardware on an aluminum spar, for example. And it is—by default—the only one of the marine sealant trio than can be safely used to bed plastic. However, silicone should not be used below the waterline. And because it depends upon mechanical compression to maintain its seal, silicone is also a poor choice for sealing hardware on a cored deck. Keep this quick review in mind when you’re considering a bedding compound: Polysulfide—a sealant suitable for bedding everything except plastic. Polyurethane—an adhesive that forms a permanent bond. Silicone—a gasket material and electrical insulator.
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