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Old 04-11-2013
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Bedding Deadlights w/ Butyl tape, DC795 and Screws

Bedding Deadlights w/ Butyl tape, DC795 and Screws

Specifically the long title:
Installing new Acrylic Deadlights w/ Butyl tape, DC795 and Trusshead screws. Is this a good, ok or poor idea?

I know this subject has been addressed a number of times but I have not seen this particular method suggested or addressed.

On my new, to me, Precision 28 I managed to crack one of my portside deadlight. I tightened a loose screw on an interior trim piece and well...

The current dead lights are once piece 68" long x 11" high tapering down to 9" high in a trapezoid shape, covering two portholes. They are screwed to the cabin with panhead blind screws and sealed with an unknown a clear sealant (silicone?).

I am having new 1/4" acrylic dead lights cut by a local glass shop using the old dead lights as templates. However I am having the new dead lights cut as two piece one for each porthole with a matching gap with a similar overlap betweeen the two. So I will have two dead lights approximately 27" x 10" and 33" x 9".

I believe this will make them easier to deal with during the installation process as well as dealing with potential expansion issues.

So next week I will be installing my dead lights and plan to use the following procedure (bear with me here):

First to deal with the acylic expansion issues I will be drilling .218 dia holes and mounting with SS #8 Trusshead screws. A #8 screw is a .16" dia. screw this will give me .058" clearance for expansion. If acrylic expands at a rate of .000039/"/degree F and my screws are spaced at 4-5" and a temperature fluctuation between New Years and July (100 degreees) I should see an expansion of .0176. So that should give me plenty of room. With Butyl tape in each of these screw holes under a truss head of .375 I should have plenty of coverage. The trusshead should also distribute the load holding forces a little better reducing the chances of deformation.

Surface preparation:
Wetsand the mounting/sealing surfaces around each port light followed by wiping down with Interlux 202 Fiberglass Solvent Wash.

Mounting:
Apply butyl tape(1/2 x 1/16 from MaineSail) on deadlights 1/16" in from the outer edge. Wrap a small bead of butyl under the head of each screw to ensure that it fills the drilled hole. Fasten the deadlights to the cabin with the trusshead screws and gently tighten them over the course of a few hours until the tape has compressed about 50%. Leaving a a nice little 1/32" sq gap to accept the Dow Corning 795 Sealant.

Sealing:
Mask the top surface of the deadlight and around leaving a 1/8" gap. Apply the Dow Corning 795 around the deadlight making a point of working it into the 1/32" gap. Lay one more bead around in the 1/8 gap and smooth it out a nice radius/fillet. Remove the tape and allow the DC795 to cure.

I have seen the 3m VHB tape and dc795 method but I like the idea of the dead lights being mechanically fastened and I like MaineSails notes and demo of the long service life of butyl. As far as the appearance of the screws I think it looks just fine. Here are Fasters, Acrylic or Polycarbonate for windows

So, is this a good, ok or poor idea?
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