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Leaking Lexan

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Bob Eischen of Toledo, Ohio asks:

"The metal and Lexan hatches on our Slocum cutter are leaking. I’ve tried a BoatLife sealant and 3M’s 4200, but without success. What kind of sealant will stop the leaks?"

Don Casey replies:

BoatLife shouldn’t be used with Lexan because the polysulfide and solvents it contains will leach out the plasticizers and make Lexan susceptible to cracking. 3M’s 4200, like other polyurethane sealants, is vulnerable to outgases from plastics like Lexan, which can compromise polyurethane’s bonding ability. The right stuff to use when rebedding polycarbonate or acrylic in a metal frame is either GE’s Silpruf SCS2000 or Dow 795 Silicone Building Sealant. Both are silicone-based adhesives formulated for bedding windows in commercial buildings.

You must first remove the old sealant with a blade and release the Lexan lens. Then remove all the old sealant from both the frame and the Lexan. When all surfaces are absolutely clean, put a bead of sealant on the bottom lip of the frame and place the Lexan on it; use spacers to maintain the distance between the outer edges of the frame and the Lexan. When this application has cured enough to hold the Lexan in place, remove the spacers and completely fill the space between the Lexan and the frame; push the tip of the sealant cartridge forward, and use a plastic spreader to dress the sealant flat. If possible, mask the frame and the Lexan lens before you start to apply sealant. Once the sealant has cured, your hatch should be secure and leak-free.

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