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Choosing the Right Sealant or Caulking

When installing a new port light, re-sealing deck fittings or plugging a hole, the most important thing to keep in mind is using the correct caulking or sealant. A good marine sealant for bedding deck fittings must be waterproof, flexible, UV resistant, and, ideally, chemical resistant. Doing the job right starts with using the right sealant. Picking the wrong sealant can cause several problems from early failure to not being able to remove a fitting, portlight hatch etc. without destroying the surface.


If you are going to be bedding plastic material or gaskets then silicone is a good solution. Silicone should only be used above the waterline and should never be used on areas that need to be sanded or painted. There are a lot of silicone products on the market, including 3M and BoatLife.


Sealants containing polyurethane are a good choice for surfaces above and below the waterline. Popular brands include 3M’s 4200 and 5200. If you need to seal a hole, then polyurethane is a good choice due to it being able to cure quickly and last nearly a decade. However, make sure to not use it on teak or other wood surfaces or from plastic to fiberglass.


Another option is marine caulk containing polysulfide like BoatLife Life-Caulk which can last for over 20 years. Products containing polysulfide and can be used above and below the waterline. Unlike polyurethane, polysulfide can be used on wood but does not work well on acrylic. It’s a great sealant for hardware as well. Make sure you apply enough of it for it to maintain a strong grip.


Whatever sealant you choose, always make sure your surfaces are clean before application and watch out for the temperature per the manufacturer's recommendations. Spend some time doing some research before you choose a caulk or sealant and avoid dangerous or costly sitautions.

Tips for Sealing On Boats


  1. Question: If a seam is too deep between wooden planks, can cotton be used to partially fill it?


    Answer: Yes, Life-Caulk or polysulfide type material is much too expensive to completely fill the seam so yes, cotton can be used.

  3. Question: Once applied can I remove 3M 5200?


    Answer: More than likely, the answer is no. There are some very strong chemicals on the market like DeBond, that you can try to use to remove 5200.

  5. Question: Can I apply new silicone sealant over an old sealant?


    Answer: It is recommended to completely remove the old silicone and clean the area before trying to add new silicone.