Sky Light Sealant

[Mysterious Ways]

I will be resealing a leaky sky light (acrylic) on my H35.5 in the spring. I was advised to use Dow Corning 795 silicone sealant because "...it has the ability to stretch or compress 50% so it is flexible enough to maintain a seal despite the thermal movement of the lens".
Has anyone used the 795? Is there other sealant that you could recommend?
Thanks!

 

[CtoSea]

Dow Corning 795 is a structural glazing sealant that is used to glaze the butt joints of mullionless window systems on high rise buildings. It has been used for a good number of years for structural glazing; in fact that is what it was developed for. I believe it would be ideal for your application.

 

[BroadReach]

The better sealant is the Dow Corning 937 which is used in the sealing of airplane cockpit windows. I have used DC795 and it is not as good as the DC937.

But and this is a big but - don't even attempt a reseal without the use of the Dow Corning OS-2 cleaner. This is a special fluid in a spray can that will clean up and prepare the surface of the sail boat deck pocket and the lexan panel insert. The OS-2 will insure the DC937 adheres to the surfaces in use.

Also do not bed or screw the panel in the pocket
The principle here is to add just enough DC937 to make a "rubber band" type of seal around the outer edge (none on the bottom) of the lexan insert.

I used a double sided tape to keep the lexan panel in place as the sealant is applied.
The lexan panel has a tendency to move around in the deck pocket and the d/s tape will hold it in place. You need a constant thickness of the "rubber band" around the edge of the lexan. The d/s tape will also keep the sealant from flowing down and into the flat of the cavity where it will try and bed the panel - which is a no no . The Lexan panel will expand at a different rate than the deck material. Bedding it in the pocket will hinder this expansion.

The d/s tape need not be very thick

I've had my HL 35 since 1999 and have reset/resealed the top panels three times. You mentioned that your stylites were "acrylic" I purchased my replacement panels (3) from Hunter and they were Lexan. I check on your "acrylic" panels.

I would not wait till spring to do the re-bed. Black is the best DC937 to use.

I tried white and it gets dirty fast - but I just checked and your in Canada - the white will blend with the snow.

Trust this will help.

RD

 

[Alan]

Yes Dow 795

That's what I used on mine. The hard part is the cleaning and sanding before you use the sealant. Be sure to use the rubber spacers between the skylight and the fiberglass opening. After pressing the lens into place the excess can be easily removed with your finger soaked with liquid soap. It leaves a factory looking finish.

 

[Neil McDonald]

Dow Corning

Just to give you some extra choice - I have successfully used Dow Corning 995!! At least we all agree on the manufacturer. It worked for me. How good can the aircraft stuff be if it has had to be redone three times since 1999?

Be warned, this can all be a very messy job. I like the tip of wiping the joint with a soapy finger.

Agree with using small self adhesive stops to hold the Lexan (or whatever) just clear of the fibreglass base. These are normally used to stop cupboard doors slamming shut. Also instead of double sided tap, I used "backer rod" - a small diameter circular foam extrusion - to stop the DC migrating below its intended position. Both available at any Canadian Tire store - a national institution that even sells the ocasional tire!!

 

[Honeyman]

Dow 795 is what Hunter used and still recommends...that should answer that. As Alan indicates, cleaning the old surface is critical. Absolutely NO old sealant should be present. I like the idea of a light sanding to prep the surface. My experience was with the fixed side lights. Use blue painter's tape to "outline" the area and keep the sealant from going everywhere while you work on it. Hunter said the longer you could let the boat sit (read: no flexing) after repair, the better. They suggested 2 weeks to allow it to cure.