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I'll be replacing one of my port lights and would like some advice on the best way to seal it. The instructions just say to use a sealant but doesn't specify anything in particular. I've read a number of bloggers who are quite vocal about not using any silicone sealant. 4200 gets a lot of votes. I'm a big fan of using butyl tape for deck fittings and was wondering if it's just as applicable for doing the port light. Thoughts?
 

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My portlights have been watertight and sound for 5 years using 3M VHB tape and Dow 795 silicone. This is basically the same combo that keeps skyscraper windows from killing pedestrians. If your lights are screwed on, the butyl will work fine but be sure to use a quality product designed for harsh environments. If I were you, though, I'd search the posts here. I am the senior designer for one of the largest skylight manufacturers in the US. I've posted instructions a few times but another fellow posted a set that was so much better than what I supplied. You won't be sorry.

Don
 

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I rebuilt my portlights by putting new plexiglass in the existing frames. I used butyl tape for the crucial seal between portlight frame and the cutout, though adhesion was handled by pop rivets, the butyl tape was only for sealant. Four portlights redone and No leaks. None.

For any application needing sealant but adhesion can be done with fasteners, I cannot imagine a better solution than butyl tape.
 

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I should add that I bedded the plexiglass in the aluminum frames with Dow 795... then bedded that assembly to the cabin cutouts with butyl tape and rivets. The Dow product has been equally effective for its purpose of adhesion and sealant.
 

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Have to agree. If the portlight has other fasteners and you just need a seal, then butyl is the way to go. If you need to seal AND glue it in place though, then something like 4200 is probably better to substitute into it.
 

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I've read a number of bloggers who are quite vocal about not using any silicone sealant.
The issue with silicone sealants is that the silicone molecules migrate through your gelcoat and make adhesion of paint and other coatings very difficult.

I should add that I bedded the plexiglass in the aluminum frames with Dow 795... then bedded that assembly to the cabin cutouts with butyl tape and rivets. The Dow product has been equally effective for its purpose of adhesion and sealant.
That approach should be fine. Just keep the silicone out of contact with your gelcoat.
 

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If these are Beckson use Dow795. Other sealants can damage the plastic. Bed-It tape will not harm the plastic but these ports being so "flexy" lend better to Dow795. You could use a combo depending upon how carefully you do the install...
 
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