How to Rebed a Leaky Deck Prism

Calypso, a traditional-looking boat, has 4 traditional-looking deck prisms. These glass fixtures in the deck let in light that’s so effective it’s hard, when the sun is shining, not to think a light has been left turned on. This is great in places like the galley and the forepeak, and perhaps more of a novelty in the 2 places in the main salon where we have them.

  Deck prism on the port side. Over the bunk project.

Deck prism on the port side. Over the bunk project.

You can see the deck crazing (an upcoming project on its own) and the fact that there are only 4 fasteners, the latter of which might be contributing to the leak situation though Jeremy’s not sold on that concept.

Still, the prism is leaking. Leaking enough that we leave a bucket under it to catch all the drips. When inspected closely, it’s clear the leak is between the bronze and the fiberglass, NOT the glass and the bronze (which makes me happy, since I rebedded THAT part a year ago!)

As we make progress on the bunk project, we’re more and more bothered by the drips.

I decided this was the weekend to tackle it. Sunday blew in clear, cold, and windy - but the key part (a rarity these days it seems) was NO RAIN.

Butyl tape is our sealant of choice for a project like this. It’s a combination of adhesive and sealant, in a very convenient tape form, that doesn’t harden and gets more adhesive over time. Clean up is far simpler than with silicone or life caulk, plus all it needs in terms of tools is a pair of scissors or a utility knife.

Step 1: Remove the prism from the deck. This means unscrewing the fasteners, carefully prying up the prism from the deck, and setting it aside.

Step 2: Create a catchment system in the now-gaping hole in the deck. Even if nobody is working directly under the space, it’s really helpful in terms of clean up to have something rigged. I use a paper towel and blue tape to hold it in place.

  Paper towel catchment clearly visible.

Paper towel catchment clearly visible.

Step 3: Remove all the old sealant from BOTH the deck AND the prism. (The catchment comes in handy as a good place to sweep it all so it does NOT go in the water.) After scraping, I go at the bronze in particular with a wire brush. If I were REALLY anal about it, I’d grab the drill, fix a wire brush to the drill, and use power.

Step 4: Clean all surfaces with a good degreaser like alcohol or acetone. You want to make sure the new sealant has no excuses not to seal.

Step 5: Apply sealant. Butyl tape has a great reputation for tacky stickiness that doesn’t harden, an important quality on a boat where things move. I applied the tape to both the deck AND the bronze, though one layer would likely have been fine.

  Butyl tape comes in a roll in different widths. This is the 3/4 inch width and it’s perfect.

Butyl tape comes in a roll in different widths. This is the 3/4 inch width and it’s perfect.

Step 6: Apply sealant to the FASTENERS. I cut a small piece of butyl tape, folded it in half, then wrapped it tightly around the head of the screw. You need to seal the fasteners because otherwise the water will just go right through the threads into the deck, which you do NOT want. I also screwed right through the tape on the deck.

Step 7: Remove protective paper from the butyl tape, then fix the prism in place.

Step 8: Screw down your fasteners! You can tighten well - no need to allow for any curing or anything before tightening down. I like seeing a small bead of tape squeeze out along the edges.

  See the small bead of butyl tape peeking out? Ahh!

See the small bead of butyl tape peeking out? Ahh!

Step 9: Clean up the paper towel and all the old sealant bits, and enjoy your leak-free prism!

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